On Aug 31, 2009 20:56 -0400, George Spelvin wrote: > >> The more I learn about storage, the more I like idea of zfs. Given the > >> subtle issues between filesystem and raid layer, integrating them just > >> makes sense. > > > > Note that all that zfs does is tell you that you already lost data (and > > then only if the checksumming algorithm would be invalid on a blank block > > being returned), it doesn't protect your data. > > Obviously, there are limits, but it does provide useful protection: > - You know where the missing data is. > - The error isn't amplified by believing corrupted metadata > - I seem to recall that ZFS does replicate metadata. ZFS definitely does replicate data. At the lowest level it has RAID-1, and RAID-Z/Z2, which are pretty close to RAID-5/6 respectively, but with the important difference that every write is a full-stripe-width write, so that it is not possible for RAID-Z/Z2 to cause corruption due to a partially-written RAID parity stripe. In addition, for internal metadata blocks there are 1 or 2 duplicate copies written to different devices, so that in case of a fatal device corruption (e.g. double failure of a RAID-Z device) the metadata tree is still intact. > - Corrupted replicas can be "scrubbed" and rewritten from uncorrupted ones. > - If you have some storage redundancy, it can try different mirrors > to get the data back. > > In particular, on a RAID-5 system, ZFS tries dropping out each data disk > in turn to see if the correct data can be reconstructed from the others > + parity. What else is interesting is that in the case of 1-4-bit errors the default checksum function can also be used as ECC to recover the correct data even if there is no replicated copy of the data. > One of ZFS's big performance problems is that currently it only checksums > the entire RAID stripe, so it always has to read every drive, and doesn't > get RAID's IOPS advantage. Or this is a drawback of the Linux software RAID because it doesn't detect the case when the parity is bad before there is a second drive failure and the bad parity is used to reconstruct the data block incorrectly (which will also go undetected because there is no checksum). Cheers, Andreas -- Andreas Dilger Sr. Staff Engineer, Lustre Group Sun Microsystems of Canada, Inc. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
