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March 05, 2013

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John Thomas

We didn't see the benefits you speak of, and not because of bulk sequential I/O. When you have more than 80GB assigned to a database's buffer cache for a 1TB database, in our case we saw about 5% of the I/O requests being met by the VFCache card.

You can be as fast as you like with an SSD cache card, but if 95% of your I/O comes from system memory, you can't help us, sorry. Our next generation of servers have something like 380GB of physical RAM...

Chuck Hollis

Hi John -- thanks for sharing your experiences.

Two things going on here (potentially).

First, you're right -- configure ginormous amounts of system RAM, and you'll get a great caching effect. That's been true for a long time.

Second, if you have an extremely random I/O pattern, caching of any sort isn't that helpful -- unless the cache size is a significant portion of the target database size.

That's part of the rationale behind all-flash arrays, such as XtremIO.

Third, we have to keep in mind that none of the server side caching today (RAM, SSD, PCIe, whatever) can safely be used for persistent writes - although good server-side caching can free up IOPS for the array to process more writes.

Customers have told me that using enormous amounts of RAM is (a) expensive, (b) can make the system unstable, and (c) can't be shared between multiple applications potentially running on the same server. There are popular exceptions (SAP's HANA comes to mind) and I'd be interested in your take as well.

Thanks for the note -- and for giving VFcache a shot!

-- Chuck

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Chuck Hollis


  • Chuck Hollis
    Chief Strategist, VMware SAS BU
    @chuckhollis

    Chuck has recently joined VMware in a new role, and is quite enthused!

    Previously, he was with EMC for 18 years, 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 Holliston, MA with his wife, three kids and four dogs when he's not travelling. In his spare time, Chuck is working on his second career as an aging rock musician.

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