ZFS ZFS send and receive won't change recordsize on existing file.

B

belon_cfy

Hi
Recently I have experienced low performance on 4k read benchmark in my VM with only 21-25MB on read, however, the read on host was saturated the SSD read which is 530MB/s during benchmark.

After investigation, I realized that the issue was due to read amplification on 128k recordsize filesystem, afterwards, I created a new VM on a 4k zfs volume and re-run the random 4k read benchmark again, the performance was aligned with the host which is > 150MB/s

I would like to convert my vm image volume from 128k recordsize to at least 16k with scarifying some read overhead(larger recordsize tend to better compression ratio and scrubbing performance), however, zfs send and receive won't do the job.

I have run few test without success
This is my test volume with the default 128k recordsize
Code: 
NAME                     USED  AVAIL    REF  VSIZE  RSIZE  COMP     RATIO  ATIME  SYNC      SNAP  LATEST SNAP                 STATUS
vol                     1.64T  1.84T   128K      -   128K  lz4      1.46x  on     standard     0
vol/test1                100M  1.84T   100M      -   128K  lz4      1.00x  on     standard     0

zdb shows the file is created on 128k recordsize.
Code: 
# zdb vol/test1

Dataset vol/test1 [ZPL], ID 151461, cr_txg 18667515, 100M, 8 objects

    Object  lvl   iblk   dblk  dsize  lsize   %full  type
         8    2   128K   128K   100M   100M  100.00  ZFS plain file


I changed the parent recordsize to 4k instead and all the subvolume will be inherited, and do the zfs send and receive.

zfs set recordsize=4k vol
zfs send vol/test1@2018-08-12_16_28 | zfs receive vol/test2


New volume recordsize is 4k
Code: 
NAME                     USED  AVAIL    REF  VSIZE  RSIZE  COMP     RATIO  ATIME  SYNC      SNAP  LATEST SNAP                 STATUS
vol                     1.64T  1.84T   128K      -     4K  lz4      1.46x  on     standard     0
vol/test2                100M  1.84T   100M      -     4K  lz4      1.00x  on     standard     1  2018-08-12_16_28


However, zdb still showing the file is 128k recordsize.
Code: 
# zdb vol/test2
Dataset vol/test2 [ZPL], ID 151698, cr_txg 18667554, 100M, 8 objects
    Object  lvl   iblk   dblk  dsize  lsize   %full  type
         8    2   128K   128K   100M   100M  100.00  ZFS plain file


The recordsize only has been changed by copying the file to the same volume again. (Object 9)
Code: 
# cp test test2
# zdb vol/test2
Dataset vol/test2 [ZPL], ID 151698, cr_txg 18667554, 202M, 9 objects

    Object  lvl   iblk   dblk  dsize  lsize   %full  type
         8    2   128K   128K   100M   100M  100.00  ZFS plain file
         9    3   128K     4K   102M   100M  100.00  ZFS plain file


How can I alter the recordsize of individual file according to the parameter in zfs during send and receive?

Thank You.
 
Well, I don't know.

primarily I would say, you have your answer: You can not.

I did not even read that it should be possible in the man page.

Off topic: In the linux man page one might read that this would be possible, where in practise it does not work this way. There it only sets the recordsize for the receiving side.

Back to topic:
Like you tested already, just copy the file after setting the desired recordsize.
 
Record size only applies to new data. When using zfs send/recv the recordsize will remain the same. You will have to use something like rsync to copy the data to get the new record size.
 
Too bad, seems there is no way to change the recordsize of file with zfs send and receive.

The downtime of VM on changing recordsize by using rsync or copy is much longer. ZFS send and receive with incremental changes impact much shorter than these 2 methods.
 
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