thinking sysadmin

There’s been a lot of noise from the storage industry about flash recently – in particular, noise from EMC and Sun, both of whom recently announced storage products using flash, EMC in January and Sun earlier this month. Below are my thoughts on what EMC and Sun are doing, as well as what NetApp might do. Since I see a fair amount of visitors from all three companies here, if I’ve got something about your employer wrong, please correct me in the comments.
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• Adam Leventhal’s Weblog: Flash, Hybrid Pools, and Future Storage – Excerpts from a forthcoming ACM article on hybrid (flash-disk) storage pools, including: “Flash should be viewed not as a replacement for existing storage, but rather as a means to enhance it. [...] By combining the use of flash as an intent-log to reduce write latency with flash as a cache to reduce read latency, we can create a system that performs far better and consumes less power than other system of similar cost.” Which perhaps could be thought of as a dig at EMC’s flash implementation, although I doubt that’s how the author intended it. (Seen at c0t0d0s0.)

• Jonathan’s Blog: Anything But a Flash in the Pan – Jonathan Schwartz on Sun’s soon-to-be-released (late this year) flash drives. Quote: “ZFS will transparently incorporate Flash into the storage hierarchy of a running system, using the microprocessor cache for the most performance sensitive tasks, DRAM for the next, then Flash, then disk (then ultimately tape).” Speaking of tiered storage, I wonder how flash drives would work within a SAM-QFS implementation – and if SAM-QFS is destined to wither in the shadow of ZFS: Sure it’s open source now, but where is it going these days?

• Chuck’s Blog: Vendor Differentiation Through Thought Leadership – (Yeah, that title made me gag, too.) Excerpt: “Sun came out loud and strong recently with characteristic unbridled enthusiasm. But, in classic Sun fashion, they missed the point entirely. It’s not just the flash drives (they’re just components, right?) it’s what you can do with them for customers. Seems that their target use case is ZFS running on a storage-oriented server. Not the most compelling use case, IMHO.” Maybe it’s just cynical old me, but I read that as “Sun has some pretty good technology with their server hardware and ZFS, Lustre and SAM-QFS; they might be able to put something together that could really hurt our margins, not just NetApp’s. Flash drives might help them do it. Uh-oh. Maybe we better hurry up and ship Maui.” The Sun/Intel PDF that Chuck links to is actually pretty interesting, although it would have been good to see some more numbers.

I just finished reading a paper presented at FAST ’08 from the University of Wisconsin, Madison (including the first and senior authors) and NetApp: Parity Lost and Parity Regained. The paper discusses the concept of parity pollution, where, in the words of the authors, “corrupt data in one block of a stripe spreads to other blocks through various parity calculations.”

With the NetApp employees as (middle) authors, the paper seems to have a slight orientation towards a NetApp perspective, but it does mention other filesystems, including ZFS both specifically and later by implication when discussing filesystems that use parental checksums, RAID and scrubbing to protect data. (Interestingly, the first specific mention of ZFS contains a gaffe where they refer to it using “RAID-4″ instead of RAID-Z.) The authors make an attempt to quantify the probability of loss or corruption – arriving at 0.486% probability per year for RAID-Scrub-Parent Checksum (implying ZFS) and 0.031% probability per year for RAID-Scrub-Block Checksum-Physical ID-Logical ID (WAFL) when using nearline drives in a 4 data disk, 1 parity disk RAID configuration and a read-write-scrub access pattern of 0.2-0.2-0.6.
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