Date: Sat, 8 Aug 2015 13:23:03 +0200 From: Willem Jan Withagen <wjw@digiware.nl> To: Konstantin Belousov <kostikbel@gmail.com> Cc: fs@freebsd.org Subject: Re: Using SSDs as swap Message-ID: <55C5E697.4080102@digiware.nl> In-Reply-To: <20150808103810.GB2072@kib.kiev.ua> References: <55C5D48E.6010605@digiware.nl> <20150808102900.GA2072@kib.kiev.ua> <20150808103810.GB2072@kib.kiev.ua>
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On 8-8-2015 12:38, Konstantin Belousov wrote: > On Sat, Aug 08, 2015 at 01:29:00PM +0300, Konstantin Belousov wrote: >> On Sat, Aug 08, 2015 at 12:06:06PM +0200, Willem Jan Withagen wrote: >>> one of the following commits just passed with this in the log, and it >>> triggered again a question I've been having for some time again already. >>> >>> ---- >>> Log: >>> Enable BIO_DELETE passthru in GELI, so TRIM/UNMAP can work as expected >>> when >>> GELI is used on a SSD or inside virtual machine, so that guest can tell >>> host that it is no longer using some of the storage. >>> ----- >>> >>> In ZFS I slice my SSD's into log and caches, but on a a server with >>> little memory (which can't be grown) I use a partion on each ssd as swap >>> as well. So swappinging does not have to seek, and has faster loading >>> time. To allocate a few GB on aan SSD to swap is not really all that >>> painfull, given current sizes, but the speed difference with regular >>> spindels is impressive. >>> >>> But the questions are: >>> 1) Does the swap driver understand that backing-store needs a TRIM? >> No. >> >>> 1a) if not would it be useful, and what would it take to implement? >> One good thing is that it is simply the question of coding: the VM >> already has a place where it informs the swap pager that the page copy >> in swap is no longer needed. this is the vm_pager_page_unswapped() call >> and swap pager method swap_pager_unswapped(). swp_pager_meta_ctl() would >> need to issue BIO_DELETE to the backing storage. >> >> On the other hand, note that this would increase the amount of work >> performed, even for the swap volumes located on the rotating media, >> which is more typical and reasonable setup. >> >> I think an implementation and a knob to turn it off, or configure per >> swap partition, would be reasonable. > > One additional thing: while BIO_DELETE is in progress, the swap block > cannot be marked free, since otherwise we could write other page and > get it obliterated with the TRIM. This can be done async, but the > consequence is that swap space would be released and usable some time > after the page-in. This will affect loads which are close to OOM. Sort of makes sense to me... I take it that BIO_DELETE fires and returns before TRIM is completed? But then the SSD accepts writes to a TRIMmed block, but then mixes this up? Possibly deleting a write to a to be trimmed block? This sort of strikes me as odd, but then I do not know the full intricate details of TRIM on SSD Would it be possible to be notified that a TRIM has completed, only then to actually free the swap sectors? And then perhaps the swap bookkeeping does not yet accommodate for a possible extra state? Speaking about blocks.... Does Swap take into account that disks could be of a sectorsize other than 512 bytes. I would guess so, since we could have a 4K disk as swap disk, and doing read-modify-write for swap is sure going to kill performance. --WjW
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