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Date: Sat, 21 Jan 2017 15:51:31 -0800
From: John-Mark Gurney <jmg@funkthat.com>
To: Aijaz Baig <aijazbaig1@gmail.com>
Cc: "Greg 'groggy' Lehey" <grog@freebsd.org>,
FreeBSD Hackers <freebsd-hackers@freebsd.org>, freebsd-scsi@freebsd.org
Subject: Re: Understanding the rationale behind dropping of "block devices"
Message-ID: <20170121235131.GF1768@funkthat.com>
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Aijaz Baig wrote this message on Mon, Jan 16, 2017 at 14:19 +0530:
> Nevertheless my question still holds. What does 'removing support for block
> device' mean in this context? Was what I mentioned earlier with regards to
> my understanding correct? Viz. all disk devices now have a character (or
> raw) interface and are no longer served via the "page cache" but rather the
> "buffer cache". Does that mean all disk accesses are now direct by passing
> the file system??
One of the other reasons block devices were removed was that if there
was a write error on the underlying device, there was no way for the
writer to know that the write failed. This could/would lead to corrupted
data which is bad.
--
John-Mark Gurney Voice: +1 415 225 5579
"All that I will do, has been done, All that I have, has not."
From owner-freebsd-scsi@freebsd.org Sun Jan 22 08:02:44 2017
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To: John-Mark Gurney <jmg@funkthat.com>
cc: Aijaz Baig <aijazbaig1@gmail.com>,
"Greg 'groggy' Lehey" <grog@freebsd.org>,
FreeBSD Hackers <freebsd-hackers@freebsd.org>, freebsd-scsi@freebsd.org
Subject: Re: Understanding the rationale behind dropping of "block devices"
In-reply-to: <20170121235131.GF1768@funkthat.com>
From: "Poul-Henning Kamp" <phk@freebsd.org>
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--------
In message <20170121235131.GF1768@funkthat.com>, John-Mark Gurney writes:
>Aijaz Baig wrote this message on Mon, Jan 16, 2017 at 14:19 +0530:
>> Nevertheless my question still holds. What does 'removing support for b=
lock
>> device' mean in this context? Was what I mentioned earlier with regards=
to
>> my understanding correct? Viz. all disk devices now have a character (o=
r
>> raw) interface and are no longer served via the "page cache" but rather=
the
>> "buffer cache". Does that mean all disk accesses are now direct by pass=
ing
>> the file system??
>
>One of the other reasons block devices were removed was that if there
>was a write error on the underlying device, there was no way for the
>writer to know that the write failed. This could/would lead to corrupted
>data which is bad.
This paper hopefully answers a lot of the questions:
https://www.usenix.org/conference/bsdcon02/rethinking-dev-and-devices-=
unix-kernel
-- =
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe =
Never attribute to malice what can adequately be explained by incompetence=
.
From owner-freebsd-scsi@freebsd.org Tue Jan 24 04:40:17 2017
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<20170121235131.GF1768@funkthat.com> <74696.1485072154@critter.freebsd.dk>
From: Aijaz Baig <aijazbaig1@gmail.com>
Date: Tue, 24 Jan 2017 10:10:14 +0530
Message-ID: <CAHB2L+fPpqCZJ_PXfgxZDS2qDAvs5M9fLRMXWg1d+4eDgqxQ9w@mail.gmail.com>
Subject: Re: Understanding the rationale behind dropping of "block devices"
To: Poul-Henning Kamp <phk@freebsd.org>
Cc: John-Mark Gurney <jmg@funkthat.com>,
"Greg 'groggy' Lehey" <grog@freebsd.org>,
FreeBSD Hackers <freebsd-hackers@freebsd.org>, freebsd-scsi@freebsd.org
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Oh thank you Poul. This would hopefully cover almost everything I need to
know!
On Sun, Jan 22, 2017 at 1:32 PM, Poul-Henning Kamp <phk@freebsd.org> wrote:
> --------
> In message <20170121235131.GF1768@funkthat.com>, John-Mark Gurney writes:
> >Aijaz Baig wrote this message on Mon, Jan 16, 2017 at 14:19 +0530:
> >> Nevertheless my question still holds. What does 'removing support for
> block
> >> device' mean in this context? Was what I mentioned earlier with regards
> to
> >> my understanding correct? Viz. all disk devices now have a character (or
> >> raw) interface and are no longer served via the "page cache" but rather
> the
> >> "buffer cache". Does that mean all disk accesses are now direct by
> passing
> >> the file system??
> >
> >One of the other reasons block devices were removed was that if there
> >was a write error on the underlying device, there was no way for the
> >writer to know that the write failed. This could/would lead to corrupted
> >data which is bad.
>
> This paper hopefully answers a lot of the questions:
>
> https://www.usenix.org/conference/bsdcon02/rethinking-dev-and-devices-
> unix-kernel
>
>
> --
> Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
> phk@FreeBSD.ORG | TCP/IP since RFC 956
> FreeBSD committer | BSD since 4.3-tahoe
> Never attribute to malice what can adequately be explained by incompetence.
>
--
Best Regards,
Aijaz Baig
From owner-freebsd-scsi@freebsd.org Wed Jan 25 10:26:43 2017
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Subject: [Bug 212454] [request] iSCSI hardware CRC32C support
Date: Wed, 25 Jan 2017 10:26:42 +0000
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D212454
--- Comment #3 from Ben RUBSON <ben.rubson@gmail.com> ---
Hi,
Some thoughts regarding this ?
Many thanks !
Ben
--=20
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You are the assignee for the bug.=
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From: bugzilla-noreply@freebsd.org
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Subject: [Bug 212454] [request] iSCSI hardware CRC32C support
Date: Wed, 25 Jan 2017 10:51:28 +0000
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D212454
--- Comment #4 from Edward Tomasz Napierala <trasz@FreeBSD.org> ---
Not sure, to be honest. I don't really plan on taking on this at this poin=
t,
due to lack of time. But I also think the PR is kind of titled wrong, for a
number of reasons. First, CRC32C is optional in iSCSI, and disabled by def=
ault
- and for a reason, it can lower performance (obviously). Second, I think =
we
actually do have CRC32C offload, along with other iSCSI offloads, in cxgbei=
.=20
And third, iSCSI doesn't implement CRC32C; it just calls the calculate_crc3=
2c()
routine.
So it's actually just about implementing optimized versions of
calculate_crc32c().
--=20
You are receiving this mail because:
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From owner-freebsd-scsi@freebsd.org Wed Jan 25 21:28:04 2017
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Subject: [Bug 212454] [request] iSCSI hardware CRC32C support
Date: Wed, 25 Jan 2017 21:28:04 +0000
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D212454
Ben RUBSON <ben.rubson@gmail.com> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|New |Closed
Resolution|--- |DUPLICATE
--- Comment #5 from Ben RUBSON <ben.rubson@gmail.com> ---
Thank you very much for your feedback !
Mmmmh unfortunately I do not run Chelsio but MLX adapters.
I agree with you, goal would be to have optimized versions of
calculate_crc32c().
I then just opened the following request : 216467
Thank you !
*** This bug has been marked as a duplicate of bug 216467 ***
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From owner-freebsd-scsi@freebsd.org Fri Jan 27 02:25:43 2017
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Subject: Re: Understanding the rationale behind dropping of "block devices"
To: Konstantin Belousov <kostikbel@gmail.com>
References: <CAHB2L+dRbX=E9NxGLd_eHsEeD0ZVYDYAx2k9h17BR0Lc=xu5HA@mail.gmail.com>
<20170116071105.GB4560@eureka.lemis.com>
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<20170116110009.GN2349@kib.kiev.ua>
Cc: Aijaz Baig <aijazbaig1@gmail.com>, "Greg 'groggy' Lehey"
<grog@freebsd.org>, FreeBSD Hackers <freebsd-hackers@freebsd.org>,
freebsd-scsi@freebsd.org
From: Julian Elischer <julian@freebsd.org>
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Date: Fri, 27 Jan 2017 10:25:32 +0800
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On 16/1/17 7:00 pm, Konstantin Belousov wrote:
> On Mon, Jan 16, 2017 at 05:20:25PM +0800, Julian Elischer wrote:
>> On 16/01/2017 4:49 PM, Aijaz Baig wrote:
>>> Oh yes I was actually running an old release inside a VM and yes I had
>>> changed the device names myself while jotting down notes (to give it a more
>>> descriptive name like what the OSX does). So now I've checked it on a
>>> recent release and yes there is indeed no block device.
>>>
>>> root@bsd-client:/dev # gpart show
>>> => 34 83886013 da0 GPT (40G)
>>> 34 1024 1 freebsd-boot (512K)
>>> 1058 58719232 2 freebsd-ufs (28G)
>>> 58720290 3145728 3 freebsd-swap (1.5G)
>>> 61866018 22020029 - free - (10G)
>>>
>>> root@bsd-client:/dev # ls -lrt da*
>>> crw-r----- 1 root operator 0x4d Dec 19 17:49 da0p1
>>> crw-r----- 1 root operator 0x4b Dec 19 17:49 da0
>>> crw-r----- 1 root operator 0x4f Dec 19 23:19 da0p3
>>> crw-r----- 1 root operator 0x4e Dec 19 23:19 da0p2
>>>
>>> So this shows that I have a single SATA or SAS drive and there are
>>> apparently 3 partitions ( or is it four?? Why does it show unused space
>>> when I had used the entire disk?)
>>>
>>> Nevertheless my question still holds. What does 'removing support for block
>>> device' mean in this context? Was what I mentioned earlier with regards to
>>> my understanding correct? Viz. all disk devices now have a character (or
>>> raw) interface and are no longer served via the "page cache" but rather the
>>> "buffer cache". Does that mean all disk accesses are now direct by passing
>>> the file system??
>> Basically, FreeBSD never really buffered/cached by device.
>>
>> Buffering and caching is done by vnode in the filesystem.
>> We have no device-based block cache. If you want file X at offset Y,
>> then we can satisfy that from cache.
>> VM objects map closely to vnode objects so the VM system IS the file
>> system buffer cache.
> This is not true.
>
> We do have buffer cache of the blocks read through the device (special)
> vnode. This is how, typically, the metadata for filesystems which are
> clients of the buffer cache, is handled, i.e. UFS msdosfs cd9600 etc.
> It is up to the filesystem to not create aliased cached copies of the
> blocks both in the device vnode buffer list and in the filesystem vnode.
>
> In fact, sometimes filesystems, e.g. UFS, consciously break this rule
> and read blocks of the user vnode through the disk cache. For instance,
> this happens for the SU truncation of the indirect blocks.
yes this caches blocks as an offset into a device, but it is still
really a
part of the system which provides caching services to vnodes.
(at least that is how it was last time I looked)
>
>> If you want device M, at offset N we will fetch it for you from the
>> device, DMA'd directly into your address space,
>> but there is no cached copy.
>> Having said that, it would be trivial to add a 'caching' geom layer to
>> the system but that has never been needed.
> The useful interpretation of the claim that FreeBSD does not cache
> disk blocks is that the cache is not accessible over the user-initiated
> i/o (read(2) and write(2)) through the opened devfs nodes. If a program
> issues such request, it indeed goes directly to/from disk driver, which
> is supplied a kernel buffer formed by remapped user pages. Note that
> if this device was or is mounted and filesystem kept some metadata in
> the buffer cache, then the devfs i/o would make the cache inconsistent.
>
>> The added complexity of carrying around two alternate interfaces to
>> the same devices was judged by those who did the work to be not worth
>> the small gain available to the very few people who used raw devices.
>> Interestingly, since that time ZFS has implemented a block-layer cache
>> for itself which is of course not integrated with the non-existing
>> block level cache in the system :-).
> We do carry two interfaces in the cdev drivers, which are lumped into
> one. In particular, it is not easy to implement mapping of the block
> devices exactly because the interfaces are mixed. If cdev disk device is
> mapped, VM would try to use cdevsw d_mmap or later mapping interfaces to
> handle user page faults, which is incorrect for the purpose of the disk
> block mapping.
>
From owner-freebsd-scsi@freebsd.org Fri Jan 27 17:44:10 2017
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Subject: [Bug 212454] [request] iSCSI hardware CRC32C support
Date: Fri, 27 Jan 2017 17:44:10 +0000
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D212454
--- Comment #6 from Ben RUBSON <ben.rubson@gmail.com> ---
Just for info, has just been implemented by Conrad :
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D216467
Really nice improvement !
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