On Wed, 29 Jun 2011, NeilBrown wrote:
It is not at all straight forward to implement.
For stripe/parity RAID, (RAID4/5/6) it is only safe to discard full stripes at
a time, and the md layer would need to keep a record of which stripes had been
discarded so that it didn't risk trusting data (and parity) read from those
stripes. So you would need some sort of bitmap of invalid stripes, and you
would need the fs to discard in very large chunks for it to be useful at all.
For copying RAID (RAID1, RAID10) you really need the same bitmap. There
isn't the same risk of reading and trusting discarded parity, but a resync
which didn't know about discarded ranges would undo the discard for you.
However, that might not necessarily be a problem; tools exist that can be
run manually (slightly fsck-like) and tell the drive which blocks can be
erased.
So: do-able, but definitely non-trivial.
Thanks very much for your response, you make some very good points.
I shall, for the time being, chop my SSDs in half and let them treat the
empty half as spare area, which should make performance degradation a
non-issue. I hope.
Cheers,
--
Tom De Mulder <tdm27@xxxxxxxxx> - Cambridge University Computing Service
+44 1223 3 31843 - New Museums Site, Pembroke Street, Cambridge CB2 3QH
-> 29/06/2011 : The Moon is Waning Crescent (18% of Full)
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