linux-ext4 - Re: [PATCH] mke2fs: restore verbose message for BLKDISCARD

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Message-ID: <5138EEBD.5000705@wwwdotorg.org>
Date:	Thu, 07 Mar 2013 12:47:09 -0700
From:	Stephen Warren <swarren@...dotorg.org>
To:	Lukáš Czerner <lczerner@...hat.com>
CC:	Theodore Ts'o <tytso@....edu>, Chris Ball <cjb@...top.org>,
	linux-ext4@...r.kernel.org, Stephen Warren <swarren@...dia.com>
Subject: Re: [PATCH] mke2fs: restore verbose message for BLKDISCARD

On 03/06/2013 12:13 AM, Lukáš Czerner wrote:
> On Tue, 5 Mar 2013, Stephen Warren wrote:
> 
>> Date: Tue,  5 Mar 2013 13:25:18 -0700
>> From: Stephen Warren <swarren@...dotorg.org>
>> To: Theodore Ts'o <tytso@....edu>
>> Cc: Chris Ball <cjb@...top.org>, linux-ext4@...r.kernel.org,
>>     Stephen Warren <swarren@...dia.com>
>> Subject: [PATCH] mke2fs: restore verbose message for BLKDISCARD
>>
>> From: Stephen Warren <swarren@...dia.com>
>>
>> mke2fs on a large slow eMMC device may appear to hang while executing
>> ioctl(BLKDISCARD). CTRL-C and CTRL-\ don't appear to respond, or respond
>> extremely slowly. -v doesn't give any hints what's happening. Only strace
>> is a clue. Make -v print some clues to make it easier to track down the
>> apparent hang.
>>
>> This change re-uses the original messages that were implemented as part
>> of 5827d24 "mke2fs support for BLKDISCARD" in order to easily re-use the
>> translations of that message. Note that this patch prints the first
>> message before executing the IOCTL, so the user is told what's going on
>> before the long wait.
>>
>> Signed-off-by: Stephen Warren <swarren@...dia.com>
> 
> Note that you've added the message only around the first 4k discard
> which is only used to test whether the device actually support the
> discard... which is not very useful and it can not even know whether
> the whole device discard will succeed/fail so it is misleading.

Oops. I certainly should looked at the code better.

> Moreover there actually is a message saying that we're "Discarding
> device blocks" and it even shows the progress. The step in which
> we're doing the discard (and update the progress) is given by
> DISCARD_STEP_MB = 2048MB. And all that in non verbose mode as well.

That said, it's that very first call to io_channel_discard() that is
hanging then; I don't see the existing message at all. With my change, I
do see the new message that I added. (So, no stdio flushing issue here).

I will go investigate why this API is hanging. Sorry for the noise.

...
> As mentioned we're doing 2GB discard steps so the problem really
> should not be on mkfs side. It is very much possible that the
> discard implementation on the eMMC is terribly slow (which does not
> surprise me). What is says about itself ?
> 
> grep . /sys/block/sda/queue/discard_*

/sys/block/mmcblk1/queue/discard_granularity
0
/sys/block/mmcblk1/queue/discard_max_bytes
512
/sys/block/mmcblk1/queue/discard_zeroes_data
1

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