From: Heinz Mauelshagen <heinzm@xxxxxxxxxx> This patch adds documentation explaining background of discard_zeroes_data issues with raid4/5/6 and the related devices_handle_discards_safely module parameter. Signed-of-by: Heinz Mauelshagen <heinzm@xxxxxxxxxx> --- Documentation/device-mapper/dm-raid.txt | 21 +++++++++++++++++++++ 1 file changed, 21 insertions(+) diff --git a/Documentation/device-mapper/dm-raid.txt b/Documentation/device-mapper/dm-raid.txt index bceb865..713739b 100644 --- a/Documentation/device-mapper/dm-raid.txt +++ b/Documentation/device-mapper/dm-raid.txt @@ -226,4 +226,25 @@ Version History 1.5.2 'mismatch_cnt' is zero unless [last_]sync_action is "check". 1.6.0 Add devices_handle_discards_safely module parameter to allow discards on disks anouncing uncertain discard_zeroes_data + (see *discard* below for more details) 1.7.0 Add support for MD RAID0 mappings + +*discard' +The MD raid4/5/6 personalities trust discard_zeroes_data ensuring +to return zeroes as block content after a discard and thus avoid +rewriting them; the zeroed content is used to reconstruct data +on a disk failure thus leading to corrupion in any reconstructed +block derived from discarded ones. + +Field reports showed, that an unknown number of disks fail supporting +discard_zeroes_data properly, hence they may return non-zero block +content after a discard. + +In order to avoid data corruption on recovery of failed RAID device +content using discarded blocks, we have to avoid using the feature +by default. + +For convenience, the module parameter devices_handle_discards_safely +allows users knowing for sure that discard_zeroes_data works properly +on all their raid disks to overwrite the default and make use +of the discard_zeroes_data optimization. -- 2.1.0 -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
