Il 2023-03-02 01:51 Roger Heflin ha scritto:
A spinning raid6 array is slow on writes (see raid6 write penalty).
Because of that the array can only do about 100 write operattions/sec.
True. But does flushing cached data really proceed in random LBA order
(as seen by HDDs), rather than trying to coalesce writes in linear
fashion?
If the disk is doing other work then it only has the extra capacity so
it could destage slower.
A lot depends on how big each chunk is. The lvmcache indicates the
smallest chunksize is 32k.
100G / 32k = 3 million, and at 100seeks/sec that comes to at least an
hour.
You are off an order of magnitude: 3 millions IOP at 100 IOPs means
~30000s, so about 9 hours.
Lvm bookkeeping has to also be written to the spinning disks I would
think, so 2 hours if the array were idle.
Throw in a 50% baseload on the disks and you get 4 hours.
Hours is reasonable.
If flushing happens in random disk order, than yes, you are bound to
wait several hours indeed.
Regards.
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Danti Gionatan
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