Friday May 29, 2015

On Self Describing Filesystem Metadata, by Darrick Wong

The following is a write-up by Oracle mainline Linux kernel engineer, Darrick Wong, providing some backround on his work on Linux FS Metadata Checksumming, which, after many years of work, will will be turned on by default in the upcoming e2fsprogs 1.43 and xfsprogs 3.2.3.

One of the bigger problems facing filesystems today is the problem of online verification of the integrity of the metadata. Even though storage bandwidth has increased considerably and (in some cases) seek times have dropped to nearly zero, the forensic work required to square a filesystem back to sense increases at least as quickly as metadata size, which scales up about as quickly as total storage capacity. Furthermore, the threat of random bit corruption in a critical piece of metadata causing unrecoverable filesystem damage remains as true as it ever was -- the author has encountered scenarios where corruption in the block usage data structure results in the block allocator crosslinking file data with metadata, which multiplies the resulting damage.

Self-describing metadata helps both the kernel and the repair tools to decide if a block actually contains the data the filesystem is trying to read. In most cases, this involves tagging each metadata block with a tuple describing the type of the block, the block number where the block lives, a unique identifier tying the block to the filesystem (typically the FS UUID), the checksum of the data in the block, and some sort of pointer to the metadata object that points to the block (the owner). For a transactional filesystem, it is useful also to record the transaction ID to facilitate analyzing where in time a corruption happened. Storing the FS UUID is useful in deciding whether an arbitrary metadata block actually belongs to this filesystem, or if it belongs to something else -- a previous filesystem or perhaps an FS image stored inside the filesystem. Given a theoretical mental model of an FS as a forest of trees all reachable by a single root, owner pointers theoretically enable a repair effort to reconstruct missing parts of the tree.

The checksum, while neither fool-proof nor tamper-proof, is usually a fast method to detect random bit corruption. While it is possible to choose stronger schemes such as sha256 (or even cryptographically signed hashes), these come with high performance and management overhead, which is why most systems choose a checksum of some sort. Both filesystems chose CRC32c, primarily for its ability to detect bit flips and the presence of hardware acceleration on a number of platforms. One area that the neither XFS nor ext4 have touched on is the topic of data checksumming. While it is technically possible to record the same self-description tuple for data blocks (btrfs stores at least the checksum), this was deliberately left out of the design for both XFS and ext4. There will be more to say about data block back-references later. First, requiring a metadata update (and log transaction) for every write of every block will have a sharply negative impact on rewrite performance. Second, some applications ensure that their internal file formats already provide the integrity data that the application requires; for them, the filesystem overhead is unnecessary. Migration of the data and its integrity information is easier when both are encapsulated in a single file. Third, performing file data integrity in userspace has the advantage that the integrity profiles can be customised for each program -- some may deem bitflip detection via CRC to be sufficient; others might want sha256 to take advantage of the reduced probability of collisions; and still more might go all the way to verification through digital signatures. There does not seem to be a pressing need to provide data block integrity specifically through the filesystem, unlike metadata, which is accessible only through the filesystem. In XFS, self-describing metadata was introduced with a new (v5) on-disk format. All existing v4 structure were enlarged to store (type, blocknr, fsuuid, owner, lsn); this allowed XFS to deploy a set of block verifiers to decide quickly if a block being read in matches what the reader expects. These verifiers also perform a quick check of the block's metadata at read and write time to detect bad metadata resulting from coding bugs. Unfortunately, it is necessary to reformat the filesystem to accomodate the resized metadata headers. The kernel and the repair tool, however, are still quick to discard broken metadata; however, as we will see, this new metadata format extension opens the door to enhanced recovery efforts.

For ext4, it was discovered that every metadata structure had sufficient room to squeeze in an extra four or two byte field to store checksum data while leaving the structure size and layout otherwise intact. This meant making a few compromises in the design -- instead of adding the 5 attributes to each block, a single 32-bit checksum is calculated over the type, blocknr, fsuuid, owner and block data; this value is then plugged into the checksum field. This scheme allows ext4 to decide if a block's contents match what we thought we were reading, but it will not enable us to reconstruct missing parts of the FS metadata object hierarchy. However, existing ext2/3/4 filesystems can be upgraded easily via tune2fs. In the near future, XFS could grow a few new features to enable an even greater level of self-directed integrity checking and repair. Inodes may soon grow parent directory pointers, which enable XFS to reconstruct directories by scanning all non-free (link count > 0) inodes in the filesystem. Similarly, a proposed block reverse-mapping btree makes it possible for XFS to rebuild a file by iterating all the rmaps looking for extent data. These two operations can even be performed online, which means that the filesystem can evolve towards self-healing abilities. Major factors blocking this development are (a) the inability to close an open file and (b) the need to shut down the allocators while we repair per-AG data. These improvements will be harder or impossible to implement for ext4, unfortunately.

The metadata checksumming features as described will be enabled by default in the respective mkfs tools as part of the next releases of e2fsprogs (1.43) and xfsprogs (3.2.3). Existing filesystems must be upgraded (ext4) or reformatted and reloaded (xfs) manually.

-- D

Further reading:

Monday Nov 17, 2014

China Linux Storage and File System (CLSF) Workshop 2014

This is a contributed post by Oracle mainline Linux kernel developer, Liu Bo, who recently presented at the 2014 China Linux Storage and File System (CLSF) Workshop.  This event was held on October 21st in Beijing. 


Xue Jiufei from Huawei showed us their work on OCFS2 of the last year, including bug fixes like "Packet loss when reconnect" and a few features like "Range lock based on DLM" and "Self-heal when fault recover".

She also mentioned that Huawei already plans to use OCFS2 inproduction.

Highlights of "range lock based on DLM", -- it's a Red-Black internal locking tree and write has higher priority but read range and write range can merge together, and it does delayed unlock which is expected to improve unlock performance.


Yu Chao from Samsung introduced F2FS update. F2FS is designed to be a flash friendly filesystem and it can overcome some problems of old flash fs, for example, snowball effect of wandering tree. The most important is F2FS is much faster than other flash filesystems, perhaps that's why it's merged into mainline so fast.

He talked about some details of F2FS including disk layout and core data structure, and his work mainly focuses on bug fixes.


I held this slot and talked about updates in the last year, such as "NO-HOLE", async metadata reclaim and the infamous bugs. And several people were very interested in how the bug was nailed down when I said that it's related to workqueue and is very difficult to reproduce.

There is an engineer from Fujitsu who worked a lot on workqueue, and he thought that the bug also should be a workqueue bug, and we had a discussion on the details behind the bug, and after he figured it out he said he would talk to workqueue's maintainer.


Memblaze is a company which focuses on flash storage. One of their engineers shared with us their product, AFA (All Flash Array). He mainly talked about the trend of current SSD-oriented file system, and they're using NVDIMM on AFA, but there are some challenges of using it on Linux, because Linux has heavy block layer and scalability problem, the bottleneck is Linux's IO latencies, context switch cost and interrupt issue because flash is very fast so there are plenty of interrupts sent to Linux.


Peng Tao from Primarydata talked about NFS update in the last year, he covered new features of NFSv42 and talked about pNFS's Flex file layout and nfsd's per-bucket spinlock.


Huawei's Hu Jianyang held this UBIFS (Unsorted Block Image filesystem) slot.  He talked about UBIFS's infrastruture and the difference with other UBI upper layer, it acts similar to FTL, for example, it can read/write/erase.  It can also do map/unmap.

UBIFS has features like static wear-leveling, transparent compression (lz4hc supported), writeback, and it supports flash of up to 32G size.

He also gave more details of UBIFS's FASTMAP feature, the normal UBIFS mount needs a time costing probe when flash media is fairly large, and fastmap addresses this scalability issue, it only scans a fixed number of blocks.

Linux Kernel Performance and 0day Testing

Fengguang Wu from Intel held this slot. He is the author of 0day test system. He said that this system runs thousands of vm test machines and tests over 400 Linux kernel git trees.

Initially this test system can only do compile/build test, and when errors occur, it will automatically try to git bisect to the buggy commit and notice the commit author and maintainer of the subsystem. And this test system is flexible, it can easily add testcases and now it supports performance test.

For filesystems, it runs popular tests like xfstests, fsmark, etc.

Compared to other test system, like Open POSIX Test Suite, this test system has much less code.

However, there are some issues, for example, random kernel config testing is not capable, because the number is huge, and the similar case is to handle filesystem's mount options and mkfs options. Fengguang said that these need developers help to filter out what option combination is needed.


Xie Liang from Xiao Mi mainly talked about their issues of using ext4 and linux block layer. They tried HBASE and others to build storage for their cloud service, like Micloud.

They found that local filesystem + bio has some problems, one is ext4's buffered IO latency is not good enough.  It always fluctuates with journal enabled. ext4 developers suggested them to use no journal and async journal, they said having journal only benifits ext4's fsck speed. Another problem is IO priority issue, the current io scheduler in linux didn't perform well on their systems, no matter cfq or deadline. But Taobao's engineer suggested to use their newly written io schedulers, an io scheduler of mixed cfq and deadline and another new scheduler based on IOPS called TPPS.

Taobao's Liu Zheng also gave an update of ext4.  Frankly there is no new kernel feaures in the last year, but they're planning some, he talked about engcryption support on ext4, project quota, data block checksum & reflink (from Oracle's Minging Cao).

Someone asked that why ext4 needs encryption, because we already have encryptionfs, why not use it instead? Zheng answered that with "Perhaps Google wants to use ext4's encryption support for chromium os user cache". [ED: see here]

Besides, e2fsprogs has a new compat feature, "sparse_super2", added by maintainer, it'll be used on SMR disk. e2fsprogs now supports metadata prefetch.

And ext4 is planning to remove old 'buffer head' code, support larger file(16TB to 1EB), more optimization towards SMR/flash device.


Asias He from OSv took this slot and this is an very interesting topic IMO. He introduced OSv's infrastruture. It uses BSD licence, its purpose is an OS for virtual machine in the cloud.  It works ina  hypervisor, and the big difference is that it has single address space. There is no kernel space or user space, there is no processes, only threads, there is no spinlock, only lock-free mutex. And it also supports zero-copy. With all of those features, it's very fast according to benchmarks of running memcached, cansandra and redis. After that we had a long discussion of "container based docker" vs "OSv", the conclusion it's well-designed , promising and more secure.

-- Liu Bo

Note: our coverage of the 2013 event is here.  Also, Robin Dong has also published a write-up of the 2014 event, here.

Tuesday Apr 01, 2014

LSF/MM 2014 and ext4 Summit Notes by Darrick Wong

This is a contributed post from Darrick Wong, storage engineer on the Oracle mainline Linux kernel team.

The following are my notes from LSF/MM 2014 and the ext4 summit, held last week in Napa Valley, CA.

  • Discussed the draft DIX passthrough interface. Based on Zach Brown's suggestions last week, I rolled out a version of the patch with a statically defined io extensions struct, and Martin Petersen said he'd try porting some existing asmlib clients to use the new interface, with a few field-enlarging tweaks. For the most part nobody objected; Al Viro said he had no problems "yet" -- but I couldn't tell if he had no idea what I was talking about, or if he was on board with the API. It was also suggested that I seek the opinion of Michael Kerrisk (the manpages maintainer) about the API. As for the actual implementation, there are plenty of holes in it that I intend to fix this week. The NFS/CIFS developers I spoke to were generally happy to hear that the storage side was finally starting to happen, and that they could get to working on the net-fs side of things now. Nicholas Bellinger noted that targetcli can create DIF disks even with the fileio backend, so he suggested I play with that over scsi_debug.

  • A large part of LSF was taken up with the discussion of how to handle the brave new world of weird storage devices. To recap: in the beginning, software had to deal with the mechanical aspects of a rotating disk; addressing had to be done in terms of cylinders, heads, and sectors (CHS). This made it difficult to innovate drive mechanics, as it was impossible to express things like variable zone density to existing software. SCSI eliminated this pain by abstracting a disk into a big tub of consecutive sectors, which simplified software quite a bit, though at some cost to performance. But most programs weren't trying to wring the last iota of performance out of disks and didn't care. So long as some attention was paid to data locality, disks performed adequately. Fast forward to 2014: now we have several different storage device classes: Flash, which has no seek penalty but prefers large writeouts; SMR drives with hard-disk seek penalties but requirements that all writes within a ~256MB zone be written in linear order; RAIDs, which by virtue of stripe geometries violate a few of the classic hard disk thinking; and NVMe devices which implement atomic read and write operations. Dave Chinner suggests that rather than retrofitting each filesystem to deal with each of these devices, it might be worth shoving all the block allocation and mapping operation down to a device mapper (dm) shim layer that can abstract away different types of storage, leaving FSes to manage namespace information. This suggestion is very attractive on a few levels: Benefits include the ability to emulate atomic read/writes with journalling, more flexible software-defined FTLs for flash and SMR, and improved communication with cloud storage systems -- Mike Snitzer had a session about dm-thinp and the proper way for FSes to communicate allocation hints to the underlying storage; this would certainly seem to fit the bill. I mentioned that Oracle's plans for cheap ext4 reflink would be trivial to implement with dm shims. Unfortunately, the devil is in the details -- when will we see code? For that reason, Ted Ts'o was openly skeptical.

  • The postgresql developers showed up to complain about stable pages and to ask for a less heavyweight fsync() -- currently, when fsync is called, it assumes that the caller wants all dirty data written out NOW, so it writes dirty pages with WRITE_SYNC, which starves reads. For postgresql this is suboptimal since fsync is typically called by the checkpointing code, which doesn't need to be fast and doesn't care if fsync writeback is not fast. There was an interlock scheduled for Thursday afternoon, but I was unable to attend. See LWN for more detailed coverage of the postgresql (and FB) sessions.

  • At the ext4 summit, we discussed a few cleanups, such as removing the use of buffer_heads and the impending removal of the ext2/3 drivers. Removing buffer_heads in the data path has the potential benefit that it'll make the transition to supporting block/sector size > page size easier, as well as reducing memory requirements (buffer heads are a heavyweight structure now). There was also the feeling that once most enterprise distros move to ext4, it will be a lot easier to remove ext3 upstream because there will be a lot more testing of the use of ext4.ko to handle ext2/3 filesystems. There was a discussion of removing ext2 as well, though that stalled on concerns that Christoph Hellwig (hch) would like to see ext2 remain as a "sample" filesystem, though Jan Kara could be heard muttering that nobody wants a bitrotten example.

  • The other major new ext4 feature discussed at the ext4 summit is per-data block metadata. This got started when Lukas Czerner (lukas) proposed adding data block checksums to the filesystem. I quickly chimed in that for e2fsck it would be helpful to have per-block back references to ease reconstruction of the filesystem, at which point the group started thinking that rather than a huge static array of block data, the complexity of a b-tree with variable key size might well be worth the effort. Then again, with all the proposed filesystem/block layer changes, Ted said that he might be open to a quick v1 implementation because the block shim layer discussed in the SMR forum could very well obviate the need for a lot of ext4 features. Time will tell; Ted and I were not terribly optimistic that any of that software is coming soon. In any case, lukas went home to refine his proposal. The biggest problem is ext4's current lack of a btree implementation; this would have to be written or borrowed, and then tested. I mentioned to him that this could be the cornerstone of reimplementing a lot of ext4 features with btrees instead of static arrays, which could be a good thing if RH is willing to spend a lot of engineering time on ext4.

  • Michael Halcrow, speaking at the ext4 summit, discussed implementing a lightweight encrypted filesystem subtree feature. This sounds a lot like ecryptfs, but hopefully less troublesome than the weird shim fs that is ecryptfs. For the most part he seemed to need (a) the ability to inject his code into the read/write path and some ability to store a small amount of per-inode encryption data. His use-case is Chrome OS, which apparently needs the ability for cache management programs to erase parts of a(nother) user's cache files without having the ability to access the file. The discussion concluded that it wouldn't be too difficult for him to start an initial implementation with ext4, but that much of this ought to be in the VFS layer.

 -- Darrick

[Ed: see also the LWN coverage of LSF/MM]

Tuesday Nov 05, 2013

CLSF & CLK 2013 Trip Report by Jeff Liu and Liu Bo

This is a contributed post from Jeff Liu, lead XFS developer for the Oracle mainline Linux kernel team, with contributions from Liu Bo, our lead BTRFS developer.

Recently, we attended the China Linux Storage and Filesystem workshop (CLSF), and the China Linux Kernel conference (CLK), which were held in Shanghai.

Here are the highlights for both events.

CLSF - 17th October

XFS update (led by Jeff Liu)

XFS keeps rapid progress with a lot of changes, especially focused on the infrastructure/performance improvements as well as  new feature development.  This can be reflected with a sample statistics among XFS/Ext4+JBD2/Btrfs via:

# git diff --stat --minimal -C -M v3.7..v3.12-rc4 -- fs/xfs|fs/ext4+fs/jbd2|fs/btrfs

XFS:       141 files changed, 27598 insertions(+), 19113 deletions(-)
Ext4+JBD2: 39 files changed,  10487 insertions(+), 5454 deletions(-)
Btrfs:     70 files changed,  19875 insertions(+), 8130 deletions(-)

  • What made up those changes in XFS?
    • Self-describing metadata(CRC32c). This is a new feature and it contributed about 70% code changes, it can be enabled via `mkfs.xfs -m crc=1 /dev/xxx` for v5 superblock.
    • Transaction log space reservation improvements. With this change, we can calculate the log space reservation at mount time rather than runtime to reduce the the CPU overhead.
    • User namespace support. So both XFS and USERNS can be enabled on kernel configuration begin from Linux 3.10. Thanks Dwight Engen's efforts for this thing.
    • Split project/group quota inodes. Originally, project quota can not be enabled with group quota at the same time because they were share the same quota file inode, now it works but only for v5 super block. i.e, CRC enabled.
    • CONFIG_XFS_WARN, an new lightweight runtime debugger which can be deployed in production environment.
    • Readahead log object recovery, this change can speed up the log replay progress significantly.
    • Speculative preallocation inode tracking, clearing and throttling. The main purpose is to deal with inodes with post-EOF space due to speculative preallocation, support improved quota management to free up a significant amount of unwritten space when at or near EDQUOT. It support backgroup scanning which occurs on a longish interval(5 mins by default, tunable), and on-demand scanning/trimming via ioctl(2).
  • Bitter arguments ensued from this session, especially for the comparison between Ext4 and Btrfs in different areas, I have to spent a whole morning of the 1st day answering those questions. We basically agreed on XFS is the best choice in Linux nowadays because:
    • Stable, XFS has a good record in stability in the past 10 years. Fengguang Wu who lead the 0-day kernel test project also said that he has observed less error than other filesystems in the past 1+ years, I own it to the XFS upstream code reviewer, they always performing serious code review as well as testing.
    • Good performance for large/small files, XFS does not works very well for small files has already been an old story for years.
    • Best choice (maybe) for distributed PB filesystems. e.g, Ceph recommends delopy OSD daemon on XFS because Ext4 has limited xattr size.
    • Best choice for large storage (>16TB). Ext4 does not support a single file more than around 15.95TB.
    • Scalability, any objection to XFS is best in this point? :)
    • XFS is better to deal with transaction concurrency than Ext4, why? The maximum size of the log in XFS is 2038MB compare to 128MB in Ext4.
  • Misc. Ext4 is widely used and it has been proved fast/stable in various loads and scenarios, XFS just need more customers, and Btrfs is still on the road to be a manhood.

Ceph Introduction (Led by Li Wang)

This a hot topic.  Li gave us a nice introduction about the design as well as their current works. Actually, Ceph client has been included in Linux kernel since 2.6.34 and supported by Openstack since Folsom but it seems that it has not yet been widely deployment in production environment.

Their major work is focus on the inline data support to separate the metadata and data storage, reduce the file access time, i.e, a file access need communication twice, fetch the metadata from MDS and then get data from OSD, and also, the small file access is limited by the network latency.

The solution is, for the small files they would like to store the data at metadata so that when accessing a small file, the metadata server can push both metadata and data to the client at the same time. In this way, they can reduce the overhead of calculating the data offset and save the communication to OSD.

For this feature, they have only run some small scale testing but really saw noticeable improvements. Test environment: Intel 2 CPU 12 Core, 64GB RAM, Ubuntu 12.04, Ceph 0.56.6 with 200GB SATA disk, 15 OSD, 1 MDS, 1 MON. The sequence read performance for 1K size files improved about 50%.

I have asked Li and Zheng Yan (the core developer of Ceph, who also worked on Btrfs) whether Ceph is really stable and can be deployed at production environment for large scale PB level storage, but they can not give a positive answer, looks Ceph even does not spread over Dreamhost (subject to confirmation). From Li, they only deployed Ceph for a small scale storage(32 nodes) although they'd like to try 6000 nodes in the future.

Improve Linux swap for Flash storage (led by Shaohua Li)

Because of high density, low power and low price, flash storage (SSD) is a good candidate to partially replace DRAM. A quick answer for this is using SSD as swap. But Linux swap is designed for slow hard disk storage, so there are a lot of challenges to efficiently use SSD for swap.

    • swap_map scan
      swap_map is the in-memory data structure to track swap disk usage, but it is a slow linear scan. It will become a bottleneck while finding many adjacent pages in the use of SSD. Shaohua Li have changed it to a cluster(128K) list, resulting in O(1) algorithm. However, this apporoach needs restrictive cluster alignment and only enabled for SSD.
    • IO pattern
      In most cases, the swap io is in interleaved pattern because of mutiple reclaimers or a free cluster is shared by all reclaimers. Even though block layer can merge interleaved IO to some extent, but we cannot count on it completely. Hence the per-cpu cluster is added base on the previous change, it can help reclaimer do sequential IO and the block layer will be easier to merge IO.
    • TLB flush:
      If we're reclaiming one active page, we should first move the page from active lru list to inactive lru list, and then reclaim the page from inactive lru to swap it out. During the process, we need to clear PTE twice: first is 'A'(ACCESS) bit, second is 'P'(PRESENT) bit. Processors need to send lots of ipi which make the TLB flush really expensive. Some works have been done to improve this, including rework smp_call_functiom_many() or remove the first TLB flush in x86, but there still have some arguments here and only parts of works have been pushed to mainline.
    • Page fault does iodepth=1 sync io, but it's a little waste if only issue a page size's IO. The obvious solution is doing swap readahead. But the current in-kernel swap readahead is arbitary(always 8 pages), and it always doesn't perform well for both random and sequential access workload. Shaohua introduced a new flag for madvise(MADV_WILLNEED) to do swap prefetch, so the changes happen in userspace API and leave the in-kernel readahead unchanged(but I think some improvement can also be done here).
  • SWAP discard
    • As we know, discard is important for SSD write throughout, but the current swap discard implementation is synchronous. He changed it to async discard which allow discard and write run in the same time. Meanwhile, the unit of discard is also optimized to cluster.
  • Misc: lock contention
    • For many concurrent swapout and swapin , the lock contention such as anon_vma or swap_lock is high, so he changed the swap_lock to a per-swap lock. But there still have some lock contention in very high speed SSD because of swapcache address_space lock.

Zproject (led by Bob Liu)

Bob gave us a very nice introduction about the current memory compression status. Now there are 3 projects(zswap/zram/zcache) which all aim at smooth swap IO storm and promote performance, but they all have their own pros and cons.
    • It is implemented based on frontswap API and it uses a dynamic allocater named Zbud to allocate free pages. Zbud means pairs of zpages are "buddied" and it can only store at most two compressed pages in one page frame, so the max compress ratio is 50%. Each page frame is lru-linked and can do shink in memory pressure. If the compressed memory pool reach its limitation, shink or reclaim happens. It decompress the page frame into two new allocated pages and then write them to real swap device, but it can fail when allocating the two pages.
  • ZRAM
    • Acts as a compressed ramdisk and used as swap device, and it use zsmalloc as its allocator which has high density but may have fragmentation issues. Besides, page reclaim is hard since it will need more pages to uncompress and free just one page. ZRAM is preferred by embedded system which may not have any real swap device. Now both ZRAM and ZSWAP are in driver/staging tree, and in the mm community there are some disscussions of merging ZRAM into ZSWAP or viceversa, but no agreement yet.
    • Handles file page compression but it is removed out of staging recently.

From industry (led by Tang Jie, LSI)

An LSI engineer introduced several new produces to us. The first is raid5/6 cards that it use full stripe writes to improve performance.

The 2nd one he introduced is SandForce flash controller, who can understand data file types (data entropy) to reduce write amplification (WA) for nearly all writes. It's called DuraWrite and typical WA is 0.5. What's more, if enable its Dynamic Logical Capacity function module, the controller can do data compression which is transparent to upper layer. LSI testing shows that with this virtual capacity enables 1x TB drive can support up to 2x TB capacity, but the application must monitor free flash space to maintain optimal performance and to guard against free flash space exhaustion. He said the most useful application is for datebase.

Another thing I think it's worth to mention is that a NV-DRAM memory in NMR/Raptor which is directly exposed to host system. Applications can directly access the NV-DRAM via a memory address - using standard system call mmap(). He said that it is very useful for database logging now. This kind of NVM produces are beginning to appear in recent years, and it is said that Samsung is building a research center in China for related produces. IMHO, NVM will bring an effect to current os layer especially on file system, e.g. its journaling may need to redesign to fully utilize these nonvolatile memory.

OCFS2 (led by Canquan Shen)

Without a doubt, HuaWei is the biggest contributor to OCFS2 in the past two years. They have posted 46 upstream patches and 39 patches have been merged. Their current project is based on 32/64 nodes cluster, but they also tried 128 nodes at the experimental stage. The major work they are working is to support ATS (atomic test and set), it can be works with DLM at the same time. Looks this idea is inspired by the vmware VMFS locking, i.e,

EXT4 (led by Zheng Liu)

Zheng Liu says ext4 keeps its stable style, so the major part is bug-fixes and cleanups while the minor is new features and improvements. He first talked about AIO writes performance gain on ext4, it makes use of extent status cache. So the problem is that they find the AIO path waiting on get_block_t(), ending up some unaccepted latencies, the solution is to batch get_block_t() with "fiemap(2) + FEMAP_FLAG_CACHE" and "ioclt(2) + EXT4_IOC_PRECACHE_EXTENT".

IOW, this just hands off latency from the kernel to the userspace.

BTRFS (led by Liu Bo)

I (Liu Bo) held the session and mainly talked about new features in the last year (2013). People are happy to see that more features are developed in btrfs, but are meanwhile confused about what btrfs wants to be -- generally speaking, as a 5-year-old FS, btrfs should try to be stable firstly anyway.

CLK - 18th October 2013

Improving Linux Development with Better Tools (Andi Kleen)

This talk focused on how to find/solve bugs along with the Linux complexity growing. Generally, we can do this with the following kind of tools:

  • Static code checkers tools. e.g, sparse, smatch, coccinelle, clang checker, checkpatch, gcc -W/LTO, stanse. This can help check a lot of things, simple mistakes, complex problems, but the challenges are: some are very slow, false positives, may need a concentrated effort to get false positives down. Especially, no static checker I found can follow indirect calls (“OO in C”, common in kernel):
    struct foo_ops {
            int (*do_foo)(struct foo *obj);
  • Dynamic runtime checkers, e.g, thread checkers, kmemcheck, lockdep. Ideally all kernel code would come with a test suite, then someone could run all the dynamic checkers.
  • Fuzzers/test suites. e.g, Trinity is a great tool, it finds many bugs, but needs manual model for each syscall. Modern fuzzers around using automatic feedback, but notfor kernel yet:
  • Debuggers/Tracers to understand code, e.g, ftrace, can dump on events/oops/custom triggers, but still too much overhead in many cases to run always during debug.
  • Tools to read/understand source, e.g, grep/cscope work great for many cases, but do not understand indirect pointers (OO in C model used in kernel), give us all “do_foo” instances:
    struct foo_ops {
          int (*do_foo)(struct foo *obj);
    } = { .do_foo = my_foo };
    That would be great to have a cscope like tool that understands this based on types/initializers

XFS: The High Performance Enterprise File System (Jeff Liu)


I gave a talk for introducing the disk layout, unique features, as well as the recent changes.   The slides include some charts to reflect the performances between XFS/Btrfs/Ext4 for small files.

About a dozen users raised their hands when I asking who has experienced with XFS. I remembered that when I asked the same question in LinuxCon/Japan, only 3 people raised their hands, but they are Chris Mason, Ric Wheeler, and another attendee.
The attendee questions were mainly focused on stability, and comparison with other file systems.

Linux Containers (Feng Gao)

The speaker introduced us that the purpose for those kind of namespaces, include mount/UTS/IPC/Network/Pid/User, as well as the system API/ABI. For the userspace tools, He mainly focus on the Libvirt LXC rather than us(LXC). Libvirt LXC is another userspace container management tool, implemented as one type of libvirt driver, it can manage containers, create namespace, create private filesystem layout for container, Create devices for container and setup resources controller via cgroup.
In this talk, Feng also mentioned another two possible new namespaces in the future, the 1st is the audit, but not sure if it should be assigned to user namespace or not. Another is about syslog, but the question is do we really need it?

In-memory Compression (Bob Liu)

Same as CLSF, a nice introduction that I have already mentioned above.

0-day Linux Kernel Performance Test (Yuanhan Liu)

Based on Fengguang Wu's 0-day autotest framework, Yuanhan Liu 0-day performance test integrates with the existing test tools and generates both ASCII and graphic results from test numbers. But it's not yet open sourced, only Intel internal, and the developers say that it's a bit difficult to make it open, because:

  1. it's not easy to setup the whole testsuite, a lot of efforts involved
  2. it needs many powerful machines on where there'll be a great number of VMs installed.

 Despite that it's not open, the framework does find bugs on various code in kernel, including btrfs, good for me :) [LB]


There were some other talks related to ACPI based memory hotplug, smart wake-affinity in scheduler etc., but my head is not big enough to record all those things.

-- Jeff Liu & Liu Bo

Wednesday May 08, 2013

Ext4 filesystem workshop report by Mingming Cao

This is a contributed post from Mingming Cao, lead ext4 developer for the Oracle mainline Linux kernel team.

I attended the second ext4 workshop hosted at the third day of Linux Collaboration Summit 2013.  Participants included Google, RedHat, SuSE, Taobao, and Lustre. We had about 2-4 hours of good discussion about the roadmap of ext4 for next year.

Ext4 write stall issue

A write Stall issue was reported by MM folks found during page claim testing over ext4. There is lock contention in JBD2 between journal commit and new transaction, resulting blocking IOs waiting for locks. More precisely it is caused by do_get_write_access() will block at lock_buffer(). The problem is nothing new should be visible in ext3 too. But new kernel becomes more visitable. Ted has proposed two fixes 1) avoid calling lock_buffer() during do_get_write_acess() 2) adjust jbd2 to manage buffer_head itself to reduce latency. Fixing in JBD2 would be a big effort. Propose 1) sounds more reasonable to work with.  The first action is to mark metadata update with RED_* to avoid the priority disorder meanwhile looking at the block IO layer and see if there is a way to move blocking IOs to a separate queue.

DIO lock contention issue

Another topic brought up is the Direct IO locking contention issue.  On DIO read side there is already no lock hold, but only for pagesize=blocksize case. There is not a fundamental issue why the no lock for direct IO read is not possible for blocksize <Pagesize -- agree we should remove this limit. On the Direct IO write side, two proposals about concurrent direct IO writes. One is based on in memory extent status tree, similar to xfs does, which allows dio write to different range of file possible. Another proposal is the general VFS solution which lock the pages in range during direct IO write. This would benefit all filesystems, but has challenge of sorting out multiple locks orders.  Jan Kara had a LSF session for this in more details. Looks like this approach is more promising.

Extent tree disk layout

There is discussion about support true 64 bit ext4 filesystem (64bit inode number and 64 bit block number -- currently 32 bit inode number and 48 bit blocknumber) in order to scales well. The ext4 on disk extent structure could be extended to support larger file, such as 64-bits physical block, bigger logical block, and using cluster-size as unit in extent.  This is easy to handle in e2fsprogs, but change on disk extent tree format is quite tricky to play well with punch hole, truncate etc., which depends on extent tree format. One solution is to add an layer of extent tree abstraction in memory, but this considered a big effort.

This was not entirely impossible.Jan Kara is working on extent tree code clean up, trying to factor out some common code first and teach the block allocation related code doesn't have to reply on on disk extent format. This is not a high priority for now.

Fallocate performance issue

A performance problem has been reported with fallocate really large file. Ext4 multile block allocator code(mballoc) currently limits how large a chunk of blocks could be allocated at a time. Should able to hack mballoc at lest 16MB at a time, instead of 2MB a time.

This brought out another related mballoc limitation. At present the mballoc normalize the request size to the nearest power of 2, up to 1MB.  The original requirement for this is for raid alignment.  If we lift up this limitation, with non normalized request size, fallocate could be 3 times faster.  Most likely we will address this quickly.

Buddy bitmap flush from mem too quickly

Under some memory pressure test, the buddy bitmap used to guide ext4 block allocation was been pushed out from memory too quickly, even though mark page dirty doesn't strong enough -- talk to mm people about interface mark page access() interface alternate, which ended with agreement to use fadvise to mark the pages as metadata.

data=guarded journaling mode

Back to ext3 time when there is no delayed allocation, the fsync() performance is badly hurt by the data=ordered mode, which forces flush out the data first (might be entire filesystem dirty data) before commit a metadata update. There is proposal of data=guarded mode which protect data inconsistency issue upon power failure, but would result in much better fsync result. The basic idea is the isize update wont be updated until the data has flushed to disk. This would drop of difference between data=writeback mode and data=ordered mode.

At the meeting this journalling mode was brought up again to see if we need this for ext4. Given ext4 implemented delayed allocation, the fsync performance was much improved (no need to flush unrelated file data), due to the benefit of delayed allocation, so performance benefit is not so obvious. But the benefit of this new journalling mode would great help 1) unwritten extent conversion issue, so that we could have full dio read no lock implementation, 2) also get ride of extra journalling mode.

ext4 filesystem mount options

There is discussion of ext4 testing cost due to many many different combination of ext4 mount options (total 70). Part of the reason is distro is trying to just maintain ext4 filesystem for all three filesystem (ext2.3.4) there is effort to test and valid the ext4 module still work as expected when mounted as ext3 with different mount options.  A few important mount options which need special care/investigate including support for indirect-based/extent-based files; support for Asynchronous journal commit; data=journal and delayed allocation exclusive issue.

So short summary of next year ext4 development is to mostly focus on reduce latency, improve performance and code reorganization.

-- Mingming Cao


The Oracle mainline Linux kernel team works as part of the Linux kernel community to develop new features and maintain existing code.

Our team is globally distributed and includes leading core kernel developers and industry veterans.

This blog is edited by James Morris <>


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