[dm-devel] dm-era over mdadm raid5 loses discard support

[vitalif at yourcmc.ru]

Hi

Just to note, the problem is solved by setting raid456.devices_handle_discard_safely=Y in kernel command line.

It seems that dm-table.c checks for blk_queue_discard() of underlying devices and RAID5 device has the QUEUE_FLAG_DISCARD unset and at the same time discard_granularity != 0. So discards actually work even without raid456.devices_handle_discard_safely=Y, but aren't passed via the DM device.

> Even worse, dm-linear has the same problem.
> 
> dm-linear over mdadm RAID5 doesn't support discard while mdadm device itself supports it.
> 
>> Hi!
>> 
>> A dm-era device created over a RAID5 array loses discard support.
>> 
>> Test script is attached.
>> 
>> If you run it on SSD you'll see something like:
>> 
>> loop0 4096
>> loop1 4096
>> loop2 4096
>> loop3 4096
>> md55 1048576
>> loop_test_era 0
>> 
>> I.e. mdadm device supports discard, but dm-era device started over it doesn't.
>> 
>> If you create a dm-era device over a normal block device without mdadm RAID5 or even over mdadm
>> RAID1 - it will preserve discard support.
>> 
>> Why? How can it be fixed?
>> 
>> --
>> With best regards,
>> Vitaliy Filippov
>> 
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> 
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