Re: [linux-lvm] lvconvert --uncache takes hours
Gionatan Danti <[email protected]>
| Newsgroups | dev.linux.lists.linux-lvm |
|---|---|
| Message-ID | <[email protected]> |
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 Supporto Tecnico Assyoma S.r.l. - www.assyoma.it email: [email protected] - [email protected] GPG public key ID: FF5F32A8 _______________________________________________ linux-lvm mailing list [email protected] https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/