[linux-lvm] Re: RAID5 + LVM stillborn

[Jason Smith]

On Tuesday 25 February 2003 09:57 pm, Joe Thornber wrote:
> As far as I'm aware the only reason LVM doesn't have raid5 is that
> nobody has written it.  What is the serialisation problem ?

Software RAID in 2.4 maintains a stripe cache that must be the same size as 
the incoming reads and writes.  Usually this is 4k for standard filesystem 
io.  But e.g. if you do a mkfs, then it can change.  When md gets a request 
for a different sized read or write, it flushes the cache and rebuilds for 
the new size.

That's a bad feature in LVM's case, because once you make a snapshot, you get 
a barrage of requests coming in at different sizes: filesystem (4096), 
snapshots (512) and I think syncs (0), so in effect there is no cache since 
it's syncing several times per second.

Like I said, the perfomance is very bad.  I oopsed a machine today by 
snapshotting an incoming FTP directory.  Sorry, I sort of threw up my hands 
earlier.  But tomorrow morning I'm going to see how repeatable it is and post 
to -dev.

Thanks.

-- 
Jason Smith
Open Enterprise Systems
Bangkok, Thailand