Messages in this thread | ![/](/images/icornerl.gif) | | Date | Mon, 26 Jun 2023 14:25:28 +0200 | Subject | Re: [dm-devel] [Question] Can DATA_DEV_BLOCK_SIZE_MIN_SECTORS be set to a smaller value | From | Zdenek Kabelac <> |
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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
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