linux-kernel - Re: Discard support (was Re: [PATCH] swap: send callback when swap slot is freed)

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <87f94c370908141855y44fe3c46xbfa155644a283ccb@mail.gmail.com>
Date:	Fri, 14 Aug 2009 21:55:36 -0400
From:	Greg Freemyer <greg.freemyer@...il.com>
To:	Chris Worley <worleys@...il.com>
Cc:	Matthew Wilcox <matthew@....cx>, Mark Lord <liml@....ca>,
	Bryan Donlan <bdonlan@...il.com>, david@...g.hm,
	Markus Trippelsdorf <markus@...ppelsdorf.de>,
	Matthew Wilcox <willy@...ux.intel.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Nitin Gupta <ngupta@...are.org>, Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-scsi@...r.kernel.org, linux-ide@...r.kernel.org,
	Linux RAID <linux-raid@...r.kernel.org>
Subject: Re: Discard support (was Re: [PATCH] swap: send callback when swap 
	slot is freed)

On Fri, Aug 14, 2009 at 8:38 PM, Chris Worley<worleys@...il.com> wrote:
> On Fri, Aug 14, 2009 at 6:30 PM, Greg Freemyer<greg.freemyer@...il.com> wrote:
>> On Fri, Aug 14, 2009 at 8:19 PM, Chris Worley<worleys@...il.com> wrote:
>>> On Fri, Aug 14, 2009 at 5:45 PM, Matthew Wilcox<matthew@....cx> wrote:
>>>> On Fri, Aug 14, 2009 at 05:21:32PM -0600, Chris Worley wrote:
>>>>> Sooner is better than waiting to coalesce.  The longer an LBA is
>>>>> inactive, the better for any management scheme.  If you wait until
>>>>> it's reused, you might as well forgo the advantages of TRIM/UNMAP.  If
>>>>> a the controller wants to coalesce, let it coalesce.
>>>>
>>>> I'm sorry, you're wrong.  There is a tradeoff point, and it's different
>>>> for each drive model.  Sending down a steady stream of tiny TRIMs is
>>>> going to give terrible performance.
>>>
>>> Sounds like you might be using junk for a device?
>>>
>>> For junk, a little coalescing may be warranted... like in the I/O
>>> schedular, but no more than 100usecs wait before posting, or then you
>>> effect high performing devices too.
>>>
>>> Chris
>>
>> Why?
>>
>> AIUI, on every write a high performing device allocates a new erase
>> block from its free lists, writes to it, and puts the now unused erase
>> block on the free list.
>
> So erase blocks are 512 bytes (if I write 512 bytes, an erase block is
> now freed)?  Not true.

Seriously, how do you  know?  Are you under NDA?

The write paper I read about typical SSD design described a partial
erase block write as:

Internal logic/micro-controller performs:

Read erase block, modify erase block, allocate new erase block, write
new erase block, free now unused old erase block, old erase block
added to a hardware erase queue the performs the actual erase in the
background at the relatively slow speed of multiple milliseconds..

The purpose of the trim/discard command being to allow the ssd to have
enough free erase blocks ready to go that the writes don't have to
stall while they wait for a erase block to pop out of the erase queue.

Greg
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/