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March 07, 2008

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Keith Stevenson

Another good paper from FAST '08 is "An Analysis of Data Corruption in the Storage Stack" http://www.usenix.org/events/fast08/tech/bairavasundaram.html

They have some very interesting observations on the expected failure modes of FC and SATA drives.

Andrés Suárez González

It's being very instructive reading your blog, thanks.

I am begining to read the "Parity lost, parity regained" article, and in the first table it seems clear that the only one doing parent checksums is ZFS. Being that the case, I suppose they implicitly haven't take into account parental checksum errors/corruption due to its implementation in ZFS: all filesystem metadata is duplicated (at least), apart of any possible lower level redundancy (RAID > 0) that is just in place (embedded on ZFS); this metadata duplication, the delay writes, the "copy on write" (the new checksum imply a cascade effect through the subdirectories until arriving to the root directory metadata, last to be written) and its embedded volume and RAID functions seem able to "guarantee" the metadata integrity in any real (tests included) scenario, if I haven't misunderstood all the information I've been reading about it.

I will finish reading the FAST article tomorrow, since I don't understand how it is possible that the embedded RAID procedure doesn't deal well with torn writes, for example.

Anyone interested on ZFS just can take a look at it, perhaps beginning at http://www.opensolaris.org/os/community/zfs/docs/zfs_last.pdf

Certainly ZFS is not so innovative in its error protection, but it is also for the masses, at least those using OpenSolaris or FreeBSD, and certainly it has another innovations.

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by Steve Todd

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