Dedup Performance Considerations

One of the major milestones for ZFS Storage appliance with 2010/Q1 is the ability to dedup data on disk. The open question is then : What performance characteristics are we expected to see from Dedup ? As Jeff says, this is the ultimate gaming ground for benchmarks. But lets have a look at the fundamentals.

ZFS Dedup Basics

Dedup code is simplistically a large hash table (the DDT). It uses a 256 bit (32 Bytes) checksum along with other metata data to identify data content. On a hash match, we only need to increase a reference count, instead of writing out duplicate data. The dedup code is integrated in the I/O pipeline and is done on the fly as part of the ZFS transaction group (see Dynamics of ZFS, The New ZFS Write Throttle ). A ZFS zpool typically holds a number of datasets : either block level LUNS which are based on ZVOL or NFS and CIFS File Shares based on ZFS filesystems. So while the dedup table is a construct associated with individual zpool, enabling of the deduplication feature is something controlled at the dataset level. Enabling of the dedup feature on a dataset, has no impact on existing data which stay outside of the dedup table. However any new data stored in the dataset will then be subject to the dedup code. To actually have existing data become part of the dedup table one can run a variant of "zfs send | zfs recv" on the datasets.

Dedup works on a ZFS block or record level. For a iSCSI or FC LUN, i.e. objects backed by ZVOL datasets, the default blocksize is 8K. For filesystems (NFS, CIFS or Direct Attach ZFS), object smaller than 128K (the default recordsize) are stored as a single ZFS block while objects bigger than the default recordsize are stored as multiple records Each record is the unit which can end up deduplicated in the DDT. Whole Files which are duplicated in many filesystems instances are expected to dedup perfectly. For example, whole DB copied from a master file are expected to falls in this category. Similarly for LUNS, virtual desktop users which were created from the same virtual desktop master image are also expected to dedup perfectly.

An interesting topic for dedup concerns streams of bytes such as a tar file. For ZFS, a tar file is actually a sequence of ZFS records with no identified file boundaries. Therefore, identical objects (files captured by tar) present in 2 tar-like byte streams might not dedup well unless the objects actually start on the same alignment within the byte stream. A better dedup ratio would be obtained by expanding the byte stream into it's constituent file objects within ZFS. If possible, the tools creating the byte stream would be well advised to start new objects on identified boundaries such as 8K.

Another interesting topic is backups of active Databases. Since database often interact with their constituent files with an identified block size, it is rather important for the deduplication effectiveness that the backup target be setup with a block size that matches the source DB block size. Using a larger block on the deduplication target has the undesirable consequence that modifications to small blocks of the source database will cause those large blocks in the backup target to appear unique and not dedup somewhat artificially. By using an 8K block size in the dedup target dataset instead of 128K, one could conceivably see up to a 10X better deduplication ratio.

Performance Model and I/O Pipeline Differences

What is the effect on performance of Dedup ? First when dedup is enabled, the checksum used by ZFS to validate the disk I/O is changed to the cryptographically strong SHA256. Darren Moffat shows in his blog that SHA256 actually runs at more than 128 MB/sec on a modern cpu. This means that less than 1 ms is consumed to checksum a 128K and less than 64 usec for an 8K unit. This cost is online incurred when actually reading or writing data to disk, an operation that is expected to take 5-10 ms; therefore the checksum generation or validation is not a source of concern.

For the read code path, very little modification should be observed. The fact that a reads happens to hit a block which is part of the dedup table is not relevant to the main code path. The biggest effect will be that we use a stronger checksum function invoked after a read I/O : at most an extra 1 ms is added to a 128K disk I/O. However if a subsequent read is for a duplicate block which happens to be in the pool ARC cache, then instead of having to wait for a full disk I/O, only a much faster copy of duplicate block will be necessary. Each filesystem can then work independently on their copy of the data in the ARC cache as is the case without deduplication. Synchronous writes are also unaffected in their interaction with the ZIL. The blocks written in the ZIL have a very short lifespan and are not subject to deduplication. Therefore the path of synchronous writes is mostly unaffected unless the pool itself ends up not being able to absorb the sustained rate of incoming changes for 10s of seconds. Similarly for asynchronous writes which interact with the ARC caches, dedup code has no affect unless the pool's transaction group itself becomes the limiting factor. So the effect of dedup will take place during the pool transaction group updates. Here is where we take all modifications that occurred in the last few seconds and atomically commit a large transaction group (TXG). While a TXG is running, applications are not directly affected except possibly for the competition for CPU cycles. They mostly continue to read from disk and do synchronous write to the zil, and asynchronous writes to memory. The biggest effect will come if the incoming flow of work exceed the capabilities of the TXG to commit data to disk. Then eventually the reads and write will be held up by the necessary write (Throttling) code preventing ZFS from consuming up all of memory .

Looking into the ZFS TXG, we have 2 operations of interest, the creation of a new data block and the simple removal (free) of a previously used block. ZFS operating under a copy on write (COW) model, any modification to an existing block actually represents both a new data block creation and a free to a previously used block (unless a snapshot was taken in which case there is no free). For file shares, this concerns existing file rewrites; for block luns (FC and iSCSI), this concerns most writes except the initial one (very first write to a logical block address or LBA actually allocates the initial data; subsequent writes to the same LBA are handled using COW). For the creation of a new application data block, ZFS will then run the checksum of the block, as it does normally and then lookup in the dedup table for a match based on that checksum and a few other bits of information. On a dedup table hit, only a reference count needs to be increased and such changes to the dedup table will be stored on disk before the TXG completes. Many DDT entries are grouped in a disk block and compression is involved. A big win occurs when many entries in a block are subject to a write match during one TXG. Then a single 1 x 16K I/O can then replace 10s of larger IOPS. As for free operations, the internals of ZFS actually holds the referencing block pointer which contains the checksum of the block being freed. Therefore there is no need to read nor recompute the checksum of the data being freed. ZFS, with checksum in hand, looks up the entry in dedup table and decrement the reference counter. If the counter is non zero then nothing more is necessary (just the dedup table sync). If the freed block ends up without any reference then it will be freed.

The DEDUP table itself an an object managed by ZFS at the pool level. The table is considered metadata and it's elements will be stored in the ARC cache. Up to 25% of memory (zfs_arc_meta_limit) can be used to store metadata. When the dedup table actually fits in memory, then enabling dedup is expected to have a rather small effect on performance. But when the table is many time greater than allotted memory, then the lookups necessary to complete the TXG can cause write throttling to be invoked earlier than the same workload running without dedup. If using an L2ARC, the DDT table represents prime objects to use the secondary cache. Note that independent of the size of the dedup table, read intensive workloads in highly duplicated environment, are expected to be serviced using fewer IOPS at lower latency than without dedup. Also note that whole filesystem removal or large file truncation are operation that can free up large quantity of data at once and when the dedup table exceeds allotted memory then those operation, which are more complex with deduplication, can then impact the amount of data going into every TXG and the write throttling behavior.

So how large is the dedup table ?

The command zdb -DD on a pool shows the size of DDT entries. In one of my experiment it reported about 200 Bytes of core memory for table entries. If each unique object is associated with 200 Bytes of memory then that means that 32GB of ram could reference 20TB of unique data stored in 128K records or more than 1TB of unique data in 8K records. So if there is a need to store more unique data than what these ratio provide, strongly consider allocating some large read optimized SSD to hold the DDT. The DDT lookups are small random IOs which are handled very well by current generation SSDs.

The first motivation to enable dedup is actually when dealing with duplicate data to begin with. If possible procedures that generate duplication could be reconsidered. The use of ZFS Clones is actually a much better way to generate logically duplicate data for multiple users in a way that does not require a dedup hash table.

But when the operating conditions does not allow the use of ZFS Clones and data is highly duplicated, then the ZFS deduplication capability is a great way to reduce the volume of stored data.

The views expressed on this blog are my own and do not necessarily reflect the views of Oracle.

Referenced Links :





Comments:

Do I read "However if a subsequent read is for a duplicate block which happens to be in the pool ARC cache ... only a much faster copy of duplicate block will be necessary" correctly as "Data is not held deduped in the ARC cache"?

I.e. using the S7000 to hold OS image in a VDI farm might dedup the OS images very effectively but would not be very effective in serving the OS images? The cache would have to be scaled linearly with the number of VMs running?

Posted by Moritz on mars 12, 2010 at 05:11 AM MET #

Moritz, That's correct.

Posted by Roch Bourbonnais on mars 12, 2010 at 10:56 AM MET #

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