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February 02, 2015


Tom K


1st, huge fan of VSAN, FYI. However, some questions regarding all-flash configs and life of the flash devices.

1/How do higher endurance cache devices minimize and extend the life of capacity tier devices in all flash config?

2/Anything to help w/WL, OP, GC or endurance/PE cycles?


Chuck Hollis

Hi Tom

All-flash VSAN uses more expensive, write-endurant cache in front of less expensive (and less-write-endurant) capacity flash. Its only purpose is to extend the life of the capacity tier. It's not about performance.

The algorithms try to keep written data in cache as long as possible, and -- when it must be evacuated to accept new writes -- it's done in a way that minimizes wear (e.g. page at a time). This is a different approach than most read/write caches that are much more aggressive at destaging data from cache.

Our experience in engineering tests is that using 10% write cache in this manner means that (a) writes are kept in cache a surprisingly long time, and (b) there end up being very minimal writes to the capacity tier.

This means that -- in most situations -- you can get away with much more cost-effective capacity flash that doesn't burn out quickly as a result.

Now, understandably, there are workloads that do nothing but write data (e.g. data stream capture and similar), and these would require a more expensive (and write endurant) capacity tier, or use magnetic disks.

Hope this helps?

-- Chuck

A VSAN patriot

Loved this piece the best
" Another thread was from a competitor around the attractiveness of data locality.

The assertion was that it made performance sense to keep data and application together on the same node, with absolutely no evidence to support the claim. "

They should just be counting their days to extinction !!!

The comments to this entry are closed.

Chuck Hollis

  • Chuck Hollis
    SVP, Oracle Converged Infrastructure Systems

    Chuck now works for Oracle, and is now deeply embroiled in IT infrastructure.

    Previously, he was with VMware for 2 years, and EMC for 18 years before that, most of them great.

    He enjoys speaking to customer and industry audiences about a variety of technology topics, and -- of course -- enjoys blogging.

    Chuck lives in Vero Beach, FL with his wife and four dogs when he's not traveling. In his spare time, Chuck is working on his second career as an aging rock musician.

    Warning: do not ever buy him a drink when there is a piano nearby.

    Note: these are my personal views, and aren't reviewed or approved by my employer.
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