On Wed, Mar 01, 2023 at 11:44:00PM +0100, Roy Sigurd Karlsbakk wrote: > Hi all > > Working with a friend's machine, it has lvmcache turned on with writeback. This has worked well, but now it's uncaching and it takes *hours*. The amount of cache was chosen to 100GB on an SSD not used for much else and the dataset that is being cached, is a RAID-6 set of 10x2TB with XFS on top. The system mainly works with file serving, but also has some VMs that benefit from the caching quite a bit. But then - I wonder - how can it spend hours emptying the cache like this? Most write caching I know of last only seconds or perhaps in really worst case scenarios, minutes. Since this is taking hours, it looks to me something should have been flushed ages ago. > > Have I (or we) done something very stupid here or is this really how it's supposed to work? It’s likely normal. HDDs stink at small random writes and RAID-6 makes this even worse. That said, I *strongly* recommend using three-disk RAID-1 for the cache, to match the redundancy of the RAID-6. With write-back caching, a failed cache will result in a corrupt and unrecoverable filesystem. -- Sincerely, Demi Marie Obenour (she/her/hers) Invisible Things Lab
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