dm raid: document RAID 4/5/6 discard support

	"repair" - Initiate a repair of the array.

	"reshape"- Currently unsupported (-EINVAL).

Discard Support

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The implementation of discard support among hardware vendors varies.

When a block is discarded, some storage devices will return zeroes when

the block is read.  These devices set the 'discard_zeroes_data'

attribute.  Other devices will return random data.  Confusingly, some

devices that advertise 'discard_zeroes_data' will not reliably return

zeroes when discarded blocks are read!  Since RAID 4/5/6 uses blocks

from a number of devices to calculate parity blocks and (for performance

reasons) relies on 'discard_zeroes_data' being reliable, it is important

that the devices be consistent.  Blocks may be discarded in the middle

of a RAID 4/5/6 stripe and if subsequent read results are not

consistent, the parity blocks may be calculated differently at any time;

making the parity blocks useless for redundancy.  It is important to

understand how your hardware behaves with discards if you are going to

enable discards with RAID 4/5/6.

Since the behavior of storage devices is unreliable in this respect,

even when reporting 'discard_zeroes_data', by default RAID 4/5/6

discard support is disabled -- this ensures data integrity at the

expense of losing some performance.

Storage devices that properly support 'discard_zeroes_data' are

increasingly whitelisted in the kernel and can thus be trusted.

For trusted devices, the following dm-raid module parameter can be set

to safely enable discard support for RAID 4/5/6:

    'devices_handle_discards_safely'

Version History

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1.0.0	Initial version.  Support for RAID 4/5/6