[linux-lvm] lvconvert --uncache takes hours

Roy Sigurd Karlsbakk roy at karlsbakk.net
Thu Mar 2 08:33:46 UTC 2023

----- Original Message -----
> From: "Roger Heflin" <rogerheflin at gmail.com>
> To: "linux-lvm" <linux-lvm at redhat.com>
> Cc: "Malin Bruland" <malin.bruland at pm.me>
> Sent: Thursday, 2 March, 2023 01:51:08
> Subject: Re: [linux-lvm] lvconvert --uncache takes hours

> On Wed, Mar 1, 2023 at 4:50 PM Roy Sigurd Karlsbakk <roy at karlsbakk.net> 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 Sigurd Karlsbakk
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