[zfs-discuss] resilvering zil/log mirror - expected behavior?

Bill McGonigle bill at bfccomputing.com
Fri Dec 16 12:08:30 EST 2011

Hi, everybody,

I replaced a zil/log (10GB) mirror device today and was surprised by the resilver behavior.  ZFS reported that it needed to resilver the entire used capacity of the zpool (1.82T), when I would have expected it to only resilver the log mirror.   

I'm not sure what the expected behavior here is.  

The actual process completed in 41 minutes, which I don't think is enough time to even read that much data from the hard drives (500GB might be possible - it's a linux md of 5400RPM 3TB disks with a LUKS wrapper).  The L2ARC is only 30GB.  Once it got rolling it was reporting resilver speeds of ~700M/s which only the SSD's can sustain:

    270G scanned out of 1.82T at 706M/s, 0h38m to go

Perhaps there are references to that much data on hard disk on the ZIL?  Ideally, I'd have expected to need only 10GB worth of resilver on the SSD's which would have just taken seconds, but probably I'm missing something about how ZFS works.  I don't even know if it's actually a problem, and if it is, it's just one of efficiency.



# zpool attach backup scsi-SATA_INTEL_SSDSA2VP0CVLC1186007R020AGN-part2 scsi-SATA_INTEL_SSDSA2VP0CVLC118600MU020AGN-part2
# zpool status backup

  pool: backup
 state: ONLINE
 scan: resilvered 0 in 0h41m with 0 errors on Fri Dec 16 10:33:57 2011

        NAME                                                   STATE     READ WRITE CKSUM
        backup                                                 ONLINE       0     0     0
          dm-0                                                 ONLINE       0     0     0
          mirror-1                                             ONLINE       0     0     0
            scsi-SATA_INTEL_SSDSA2VP0CVLC1186007R020AGN-part2  ONLINE       0     0     0
            scsi-SATA_INTEL_SSDSA2VP0CVLC118600MU020AGN-part2  ONLINE       0     0     0
          scsi-SATA_TS64GSSD25S-M_20110211405406066105-part2   ONLINE       0     0     0

errors: No known data errors

Bill McGonigle, Owner   
BFC Computing, LLC       
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