Re: [linux-lvm] Testing ThinLVM metadata exhaustion

[Gionatan Danti <g.danti@assyoma.it>]

Il 18-04-2016 16:25 Gionatan Danti ha scritto:
> Hi all,
> I'm testing the various metadata exhaustion cases and how to cope with
> them. Specifically, I would like to fully understand what to expect
> after a metadata space exhaustion and the relative check/repair. To
> such extents, metadata autoresize is disabled.
> 
> I'm using a fully updated CentOS 6.7 x84_64 virtual machine, with a
> virtual disk (vdb) dedicated to the thin pool / volumes. This is what
> pvs reports:
> 
> PV         VG          Fmt  Attr PSize  PFree
> /dev/vda2  vg_hvmaster lvm2 a--  63.51g    0
> /dev/vdb   vgtest      lvm2 a--  32.00g    0
> 
> I did the following operations:
> vgcreate vgtest /dev/vdb
> lvcreate --thin vgtest/ThinPool -L 1G 	# 4MB tmeta
> lvchange -Zn vgtest
> lvcreate --thin vgtest/ThinPool --name ThinVol -V 32G
> lvresize vgtest/ThinPool -l +100%FREE # 31.99GB, 4MB tmeta, not resized
> 
> With 64 KB chunks, the 4 MB tmeta volume is good for mapping ~8 GB, so
> any other writes trigger a metadata space exhaustion. Then, I did:
> 
> a) a first 8 GB write to almost fill the entire metadata space:
> [root@hvmaster ~]# dd if=/dev/zero of=/dev/vgtest/ThinVol bs=1M 
> count=8192
> 8192+0 records in
> 8192+0 records out
> 8589934592 bytes (8.6 GB) copied, 101.059 s, 85.0 MB/s
> [root@hvmaster ~]# lvs -a
>   LV               VG          Attr       LSize  Pool     Origin Data%
>  Meta%  Move Log Cpy%Sync Convert
>   lv_root          vg_hvmaster -wi-ao---- 59.57g
> 
>   lv_swap          vg_hvmaster -wi-ao----  3.94g
> 
>   ThinPool         vgtest      twi-aot-M- 31.99g                 21.51  
> 92.09
>   [ThinPool_tdata] vgtest      Twi-ao---- 31.99g
> 
>   [ThinPool_tmeta] vgtest      ewi-ao----  4.00m
> 
>   ThinVol          vgtest      Vwi-a-t--- 32.00g ThinPool        23.26
> 
>   [lvol0_pmspare]  vgtest      ewi-------  4.00m
> [root@hvmaster ~]# thin_dump /dev/mapper/vgtest-ThinPool_tmeta
> <superblock uuid="" time="0" transaction="1" data_block_size="128"
> nr_data_blocks="524096">
>   <device dev_id="1" mapped_blocks="121968" transaction="0"
> creation_time="0" snap_time="0">
>     <range_mapping origin_begin="0" data_begin="0" length="121968" 
> time="0"/>
>   </device>
> </superblock>
> 
> b) a second non-synched 16 GB write to totally trash the tmeta volume:
> # Second write
> [root@hvmaster ~]# dd if=/dev/zero of=/dev/vgtest/ThinVol bs=1M 
> count=8192
> 8192+0 records in
> 8192+0 records out
> 8589934592 bytes (8.6 GB) copied, 101.059 s, 85.0 MB/s
> [root@hvmaster ~]# lvs -a
>   LV               VG          Attr       LSize  Pool     Origin Data%
>  Meta%  Move Log Cpy%Sync Convert
>   lv_root          vg_hvmaster -wi-ao---- 59.57g
> 
>   lv_swap          vg_hvmaster -wi-ao----  3.94g
> 
>   ThinPool         vgtest      twi-aot-M- 31.99g                 21.51  
> 92.09
>   [ThinPool_tdata] vgtest      Twi-ao---- 31.99g
> 
>   [ThinPool_tmeta] vgtest      ewi-ao----  4.00m
> 
>   ThinVol          vgtest      Vwi-a-t--- 32.00g ThinPool        23.26
> 
>   [lvol0_pmspare]  vgtest      ewi-------  4.00m
> [root@hvmaster ~]# thin_dump /dev/mapper/vgtest-ThinPool_tmeta
> <superblock uuid="" time="0" transaction="1" data_block_size="128"
> nr_data_blocks="524096">
>   <device dev_id="1" mapped_blocks="121968" transaction="0"
> creation_time="0" snap_time="0">
>     <range_mapping origin_begin="0" data_begin="0" length="121968" 
> time="0"/>
>   </device>
> </superblock>
> 
> c) a third, synched 16 GB write to see how the system behave with
> fsync-rich filling:
> [root@hvmaster ~]# dd if=/dev/zero of=/dev/vgtest/ThinVol bs=1M
> count=16384 oflag=sync
> dd: writing `/dev/vgtest/ThinVol': Input/output error
> 7624+0 records in
> 7623+0 records out
> 7993294848 bytes (8.0 GB) copied, 215.808 s, 37.0 MB/s
> [root@hvmaster ~]# lvs -a
>   Failed to parse thin params: Error.
>   Failed to parse thin params: Error.
>   Failed to parse thin params: Error.
>   Failed to parse thin params: Error.
>   LV               VG          Attr       LSize  Pool     Origin Data%
>  Meta%  Move Log Cpy%Sync Convert
>   lv_root          vg_hvmaster -wi-ao---- 59.57g
> 
>   lv_swap          vg_hvmaster -wi-ao----  3.94g
> 
>   ThinPool         vgtest      twi-aot-M- 31.99g                 21.51  
> 92.09
>   [ThinPool_tdata] vgtest      Twi-ao---- 31.99g
> 
>   [ThinPool_tmeta] vgtest      ewi-ao----  4.00m
> 
>   ThinVol          vgtest      Vwi-a-t--- 32.00g ThinPool
> 
>   [lvol0_pmspare]  vgtest      ewi-------  4.00m
> [root@hvmaster ~]# thin_dump /dev/mapper/vgtest-ThinPool_tmeta
> <superblock uuid="" time="0" transaction="1" data_block_size="128"
> nr_data_blocks="524096">
> metadata contains errors (run thin_check for details).
> perhaps you wanted to run with --repair
> 
> It is the last scenario (c) that puzzle me: rebooting the machine left
> the thinpool inactive and inactivable (as expected), but executing
> lvconvert --repair I can see that _all_ metadatas are gone (the pool
> seems empty). Is that the expected behavior?
> 
> Even more puzzling (for me) is that by skipping test a and b, and
> going directly for c, I have a different behavior: the metadata volume
> is (rightfully) completely filled, and the thin pool went in read-only
> mode. Again, it that the expected behavior?
> 
> Regards.

Hi all,
doing more tests I noticed that when "catastrophic" (non recoverable) 
metadata loss happens, dmesg logs the following lines:

device-mapper: block manager: validator mismatch (old=sm_bitmap vs 
new=btree_node) for block 429
device-mapper: space map common: unable to decrement a reference count 
below 0
device-mapper: thin: 253:4: metadata operation 'dm_thin_insert_block' 
failed: error = -22

During "normal" metadata exhaustion (when the pool can recover), the 
first two lines are not logged at all. Moreover, the third line reports 
error = -28, rather than error = -22 as above.

I also tested the latest RHEL 7.2 and I can not reproduce the error 
above: metadata exhaustion always seems to be managed in a graceful (ie: 
recoverable) manner.

I am missing something?
Thanks.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
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