Re: fsck, raid reconstruction & bad bad 2.4.3

[Jesse Pollard <jesse@cats-chateau.net>]

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

Any opinions expressed are solely my own.