[linux-lvm] lvconvert --uncache takes hours

[Gionatan Danti]

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.

-- 
Danti Gionatan
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