linux-kernel - Re: [dm-devel] [Question] Can DATA_DEV_BLOCK_SIZE_MIN

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Date:   Mon, 26 Jun 2023 14:25:28 +0200
From:   Zdenek Kabelac <zkabelac@...hat.com>
To:     Li Lingfeng <lilingfeng3@...wei.com>, dm-devel@...hat.com
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [dm-devel] [Question] Can DATA_DEV_BLOCK_SIZE_MIN_SECTORS be set
 to a smaller value

Dne 26. 06. 23 v 14:19 Li Lingfeng napsal(a):
> Hello:
>
> Recently, I found that the used space of the thin-pool will keep rising if I 
> use dm-thin as follow:
>
> // create dm-thin
> dmsetup create linear_1 --table "0 2097152 linear /dev/sdc 0"
> dmsetup create linear_2 --table "0 16777216  linear /dev/sdc 2097153"
> dd if=/dev/zero of=/dev/mapper/linear_1 bs=4096 count=1
> dmsetup create pool --table "0 16777216 thin-pool /dev/mapper/linear_1 
> /dev/mapper/linear_2 128 0 1 skip_block_zeroing"
> dmsetup message /dev/mapper/pool 0 "create_thin 0"
> dmsetup create thin --table "0 14680064 thin /dev/mapper/pool 0"
>
> // mkfs and mount with discard
> mkfs.ext4 /dev/mapper/thin
> mount /dev/mapper/thin /mnt/test -o discard
> cd /mnt/test
>
> // create a file(17KB)
> dd if=/dev/random of=testfile bs=1k count=17 oflag=direct
> sync
>
> // truncate the file and write it for many times
> dd if=/dev/random of=testfile bs=1k count=17 oflag=direct
> sync
> ...
>
> Ext4 will issue discard IO to dm-thin when truncating file. However, 
> DATA_DEV_BLOCK_SIZE_MIN_SECTORS is set as 64KB which means the discard 
> covers less than a block when I truncating a 17KB file. As the result of it, 
> discard bio will end in process_discard_bio(), and more and more blocks will 
> leak.
>
> I'm curious about the reason behind setting DATA_DEV_BLOCK_SIZE_MIN_SECTORS 
> to 64KB. Is there any specific consideration for this? Would it be possible 
> to set this minimum limit to a smaller value, such as 4KB? 


Hi


The minimal chunksize for thin-pool was selected to  64K - smaller value would 
cause major expansion in the used 'metadata' size and further slowdown of 
metadata manipulation - so the 64K was set as minimal size.

Regards


Zdenek