linux-kernel - Re: [PATCH] scsi: sd: Keep the discard mode stable

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Message-Id: <3F36E434-F2D4-457C-860C-4CA31EAE99DC@smartx.com>
Date: Wed, 19 Jun 2024 14:58:59 +0800
From: Li Feng <fengli@...rtx.com>
To: Benjamin Block <bblock@...ux.ibm.com>
Cc: "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
 linux-kernel <linux-kernel@...r.kernel.org>,
 "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
 "Martin K. Petersen" <martin.petersen@...cle.com>
Subject: Re: [PATCH] scsi: sd: Keep the discard mode stable



> 2024年6月18日 16:47，Benjamin Block <bblock@...ux.ibm.com> 写道：
> 
> On Tue, Jun 18, 2024 at 11:06:13AM +0800, Li Feng wrote:
>>> 2024年6月18日 00:26，Benjamin Block <bblock@...ux.ibm.com> 写道：
>>> On Sat, Jun 15, 2024 at 12:03:47AM +0800, Li Feng wrote:
>>>> There is a scenario where a large number of discard commands
>>>> are issued when the iscsi initiator connects to the target
>>>> and then performs a session rescan operation. 
>>> 
>>> Is this with just one specific target implementation? This sounds like a
>>> broken/buggy target, or is there a reason why this happens in general?
>>> 
>>> And broken target sounds like device quirk, rather than impacting every
>>> possible target.
>> 
>> This is a common problem. Before sending a rescan, discard has been 
>> negotiated to UNMAP. After the rescan, there will be a short window for 
>> it to become WS16, and then it will immediately become UNMAP. 
>> However, during this period, a small amount of discard commands 
>> may become WS16, resulting in a strange problem.
> 
> Ok, interesting. Do you know why this short window happens? 

I have explained it in other emails, you can read them.

Thanks,
Li

> 
>>>> There is a time
>>>> window, most of the commands are in UNMAP mode, and some
>>>> discard commands become WRITE SAME with UNMAP.
>>>> 
>>>> The discard mode has been negotiated during the SCSI probe. If
>>>> the mode is temporarily changed from UNMAP to WRITE SAME with
>>>> UNMAP, IO ERROR may occur because the target may not implement
>>>> WRITE SAME with UNMAP. Keep the discard mode stable to fix this
>>>> issue.
>>>> 
>>>> Signed-off-by: Li Feng <fengli@...rtx.com>
>>>> ---
> 
> -- 
> Best Regards, Benjamin Block        /        Linux on IBM Z Kernel Development
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