----- Original Message ----- > From: "Roger Heflin" <rogerheflin@xxxxxxxxx> > To: "linux-lvm" <linux-lvm@xxxxxxxxxx> > Cc: "Malin Bruland" <malin.bruland@xxxxx> > Sent: Thursday, 2 March, 2023 01:51:08 > Subject: Re: lvconvert --uncache takes hours > On Wed, Mar 1, 2023 at 4:50 PM Roy Sigurd Karlsbakk <roy@xxxxxxxxxxxxx> 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? >> >> Vennlig hilsen >> >> roy > > 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. About 100 writes/second per data drive, that is. md parallilses I/O well. > If the disk is doing other work then it only has the extra capacity so > it could destage slower. The system was mostly idle. > 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. Those 100GB was on SSD, not spinning rust. Last I checked, that was the whole point with caching. > Lvm bookkeeping has to also be written to the spinning disks I would > think, so 2 hours if the array were idle. erm - why on earth would you do writes to hdd if you're caching it? > Throw in a 50% baseload on the disks and you get 4 hours. > > Hours is reasonable. As I said, the system was idle. Vennlig hilsen roy -- Roy Sigurd Karlsbakk (+47) 98013356 -- I all pedagogikk er det essensielt at pensum presenteres intelligibelt. Det er et elementært imperativ for alle pedagoger å unngå eksessiv anvendelse av idiomer med xenotyp etymologi. I de fleste tilfeller eksisterer adekvate og relevante synonymer på norsk. _______________________________________________ linux-lvm mailing list linux-lvm@xxxxxxxxxx https://listman.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
