Messages in this thread | | | From | Jesse Pollard <> | Subject | Re: fsck, raid reconstruction & bad bad 2.4.3 | Date | Sun, 15 Apr 2001 21:51:52 -0500 |
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On Sun, 15 Apr 2001, Jonathan Lundell wrote: >At 9:23 PM -0500 2001-04-15, Jesse Pollard wrote: >> >b) wait with fsck until rebuild is fixed >> >>Depends on your definition of "fixed". The most I can see to fix is >>reduce the amount of continued update in favor of updating those blocks >>being read (by fsck or anything else). This really ought to be a runtime >>configuration option. If it is set to 0, then no automatic repair would >>be done. > >The original post was referring to RAID 1; there's no repair necessary at >the RAID level to give fsck the correct data. Seems to me the basic problem >here is that the RAID re-sync is supposed to be throttling back to allow other >activity to run, but that in the case of fsck the other activity is still >slower by a large factor (compared to no RAID re-sync).
If I've got the numbering right; 0 - concatenated stripes => no sync required 1 - mirrored => resync required a: which drive has the correct info? b: having determined that, read the correct block, it must now be written to the mirror. all others => resync required (rebuild possible bad block)
>Is this a pathological case because of the way fsck does business, or does >the RAID re-sync affect any disk-bound process that severely?
My experience has been with hardware raid, and even then there has been a 1-5% decrease in I/O during resync (not accurately measured - fsck took longer, and then only when the channel is maxed out -- otherwise the 1-5% is not visible; filesystem was 3 IRIX efs, spread across two raid luns).
fsck is particularly bad, since nearly every read instigates a write to the mirror drive. Fsck can then modify the block and write it back, causing two more writes; for a total of 3 writes for a read (worst case).
It does mean that when fsck finishes, MOST of the re-sync will be finished. All of the metadata will be synced, and only file data blocks will remain.
-- ------------------------------------------------------------------------- Jesse I Pollard, II Email: jesse@cats-chateau.net
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