Saturday Jan 10, 2009

ZIL, SSD, and Other Fun Acronyms

The ZFS Intent Log (or ZIL) is always written to non-volatile storage. The ZIL allows the file system to recover from crashes without data loss. In a 7000 Series with Write Optimized SSD, the ZIL is stored on the Write Optimized SSD, otherwise it is stored in the disk pool. Either way, it is also available in system memory. The ZIL flushes to the disk pool every once in awhile (this is called a Transaction Group Commit).

In a 7410 cluster, if a fail over occurs under normal conditions the pool is imported by the alternate node, the ZIL is replayed against the pool, and the pool is online and ready. You can think of the Write Optimized  SSD and ZIL as our NVRAM if that helps, but we don't need batteries.

If the ZIL is stored on a single SSD, and that device fails, the system has a window to flush the ZIL from memory to disk (the Transaction Group Commit I mentioned earlier).  Typically in the 7000 Series, this flush happens every 1-5 seconds, but it can take up to 30 seconds on an extremely busy system.  Once the data is flushed from memory to disk, the system will use the disk pool to store the ZIL for the next transaction group. This window is the only time in a 7000 series where there is a chance for data loss. We mitigate this risk by mirroring the Write Optimized SSD's in the system.

ZFS performance on asynchronous writes is good and SSD is not required in these configurations (although it will help improve performance and is recommended) however in configurations that require synchronous writes (many iSCSI configurations, NFS with O_DSYNC etc) Write SSD is almost mandatory.

Write SSD Sizing Rules of Thumb:

-Each device supports about 9000-10000 Write IOPS (Sequential writes stream directly to disk for better performance)
-If devices are mirrored, they only count for 1x Write IOPS (ie two devices at 9000 IOPS each when mirrored together support 9000 IOPS total)
-If aiming to support No Single Point of Failure configurations, more trays with less SSD's per tray will have higher usable capacities. Clusters will only allow SSD in pairs.

 EOF

Friday Jan 09, 2009

Blogging about Wikis about Blogs...

Since we launched the 7000 Series and revealed Fishworks to the world, there has been a lot of questions both inside (via our overflowing internal mailing list) and outside Sun.  I've personally been involved with presenting the product dozens of times to staff and customers, and usually feel like a broken record as I share the answers to the usual questions, so I decided to create a central repository of information on the Fishworks wiki to help share the answers more freely. 

The first component of this was an FAQ.  It works, and contains a lot of useful stuff, but I was finding that the answers to many questions were actually in blogs written by the Fishworks team, and various other engineers within and outside sun.  The next step was to create a central repository of links to these useful blog entries where people could go to find answers to their questions.  I finished creating it the other day and have been rearranging and adding entries.  It's not perfect, and it may never be, but now is the time to reveal FishBlog Central, and I thought what better way to reveal a wiki about blogs than to blog about it. 

If you have some information or a link to add to either the FAQ or FishBlog Central, please share it so that it can be shared with the world.

EOF

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This is the weblog for Ryan Matthews, a sales consultant at Oracle specializing in the ZFS Storage Appliance. It is the home to information on sizing and much more.

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