From: "Austin S. Hemmelgarn" <ahferroin7@gmail.com>
To: Andrei Borzenkov <arvidjaar@gmail.com>,
Robert White <rwhite@pobox.com>,
Btrfs BTRFS <linux-btrfs@vger.kernel.org>
Subject: Re: What if TRIM issued a wipe on devices that don't TRIM?
Date: Fri, 7 Dec 2018 07:18:43 -0500 [thread overview]
Message-ID: <62109e89-331d-a5ea-456a-a79bc1a31451@gmail.com> (raw)
In-Reply-To: <90f90edd-c22c-b36e-4394-5a17b2a986ec@gmail.com>
On 2018-12-06 23:09, Andrei Borzenkov wrote:
> 06.12.2018 16:04, Austin S. Hemmelgarn пишет:
>>
>> * On SCSI devices, a discard operation translates to a SCSI UNMAP
>> command. As pointed out by Ronnie Sahlberg in his reply, this command
>> is purely advisory, may not result in any actual state change on the
>> target device, and is not guaranteed to wipe the data. To actually wipe
>> things, you have to explicitly write bogus data to the given regions
>> (using either regular writes, or a WRITESAME command with the desired
>> pattern), and _then_ call UNMAP on them.
>
> WRITE SAME command has UNMAP bit and depending on device and kernel
> version kernel may actually issue either UNMAP or WRITE SAME with UNMAP
> bit set when doing discard.
>
Good to know. I've not looked at the SCSI code much, and actually
didn't know about the UNMAP bit for the WRITE SAME command, so I just
assumed that the kernel only used the UNMAP command.
prev parent reply other threads:[~2018-12-07 12:18 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-12-06 6:11 What if TRIM issued a wipe on devices that don't TRIM? Robert White
2018-12-06 6:26 ` Roman Mamedov
2018-12-06 10:04 ` ronnie sahlberg
2018-12-06 13:04 ` Austin S. Hemmelgarn
2018-12-07 4:09 ` Andrei Borzenkov
2018-12-07 12:18 ` Austin S. Hemmelgarn [this message]
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