Re: light weight write barriers

From: "杨苏立 Yang Su Li" <suli-hcNo3dDEHLuVc3sceRu5cw@public.gmane.org>
To: Vladislav Bolkhovitin <vst-d+Crzxg7Rs0@public.gmane.org>
Cc: linux-fsdevel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	General Discussion of SQLite Database
	<sqlite-users-CzDROfG0BjIdnm+yROfE0A@public.gmane.org>,
	Theodore Ts'o <tytso-3s7WtUTddSA@public.gmane.org>,
	linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	Richard Hipp <drh-X1OJI8nnyKUAvxtiuMwx3w@public.gmane.org>
Subject: Re: light weight write barriers
Date: Sat, 10 Nov 2012 22:25:07 -0600	[thread overview]
Message-ID: <CABK4GYMmigmi7YM9A5Aga21ZWoMKgUe3eX-AhPzLw9CnYhpcGA@mail.gmail.com> (raw)
In-Reply-To: <508B3EED.2080003-d+Crzxg7Rs0@public.gmane.org>

On Fri, Oct 26, 2012 at 8:54 PM, Vladislav Bolkhovitin <vst-d+Crzxg7Rs0@public.gmane.org> wrote:

>
> Theodore Ts'o, on 10/25/2012 01:14 AM wrote:
>
>> On Tue, Oct 23, 2012 at 03:53:11PM -0400, Vladislav Bolkhovitin wrote:
>>
>>> Yes, SCSI has full support for ordered/simple commands designed
>>> exactly for that task: to have steady flow of commands even in case
>>> when some of them are ordered.....
>>>
>>
>> SCSI does, yes --- *if* the device actually implements Tagged Command
>> Queuing (TCQ).  Not all devices do.
>>
>> More importantly, SATA drives do *not* have this capability, and when
>> you compare the price of SATA drives to uber-expensive "enterprise
>> drives", it's not surprising that most people don't actually use
>> SCSI/SAS drives that have implemented TCQ.
>>
>
> What different in our positions is that you are considering storage as
> something you can connect to your desktop, while in my view storage is
> something, which stores data and serves them the best possible way with the
> best performance.
>
> Hence, for you the least common denominator of all storage features is the
> most important, while for me to get the best of what possible from storage
> is the most important.
>
> In my view storage should offload from the host system as much as
> possible: data movements, ordered operations requirements, atomic
> operations, deduplication, snapshots, reliability measures (eg RAIDs), load
> balancing, etc.
>
> It's the same as with 2D/3D video acceleration hardware. If you want the
> best performance from your system, you should offload from it as much as
> possible. In case of video - to the video hardware, in case of storage - to
> the storage. The same as with video, for storage better offload - better
> performance. On hundreds of thousands IOPS it's clearly visible.
>
> Price doesn't matter here, because it's completely different topic.
>
>
>  SATA's Native Command
>> Queuing (NCQ) is not equivalent; this allows the drive to reorder
>> requests (in particular read requests) so they can be serviced more
>> efficiently, but it does *not* allow the OS to specify a partial,
>> relative ordering of requests.
>>
>
> And so? If SATA can't do it, does it mean that nobody else can't do it
> too? I know a plenty of non-SATA devices, which can do the ordering
> requirements you need.
>

I would be very much interested in what kind of device support this kind of
"topological order", and in what settings they are typically used.

Does modern flash/SSD (esp. which are used on smartphones) support this?

If you could point me to some information about this, that would be very
much appreciated.

Thanks a lot!

Suli

>
> Vlad
>
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