On Wed, 1 Oct 2014 09:32:37 -0400 Mike Snitzer <snitzer@xxxxxxxxxx> wrote: > On Tue, Sep 30 2014 at 10:56pm -0400, > NeilBrown <neilb@xxxxxxx> wrote: > > > On Wed, 24 Sep 2014 13:02:28 +0200 Heinz Mauelshagen <heinzm@xxxxxxxxxx> > > wrote: > > > > > > > > Martin, > > > > > > thanks for the good explanation of the state of the discard union. > > > Do you have an ETA for the 'zeroout, deallocate' ... support you mentioned? > > > > > > I was planning to have a followup patch for dm-raid supporting a dm-raid > > > table > > > line argument to prohibit discard passdown. > > > > > > In lieu of the fuzzy field situation wrt SSD fw and discard_zeroes_data > > > support > > > related to RAID4/5/6, we need that in upstream together with the initial > > > patch. > > > > > > That 'no_discard_passdown' table line can be added to dm-raid RAID4/5/6 > > > table > > > lines to avoid possible data corruption but can be avoided on RAID1/10 > > > table lines, > > > because the latter are not suffering from any discard_zeroes_data flaw. > > > > > > > > > Neil, > > > > > > are you going to disable discards in RAID4/5/6 shortly > > > or rather go with your bitmap solution? > > > > Can I just close my eyes and hope it goes away? > > > > The idea of a bitmap of uninitialised areas is not a short-term solution. > > But I'm not really keen on simply disabling discard for RAID4/5/6 either. It > > would mean that people with good sensible hardware wouldn't be able to use > > it properly. > > > > I would really rather that discard_zeroes_data were only set on devices where > > it was actually true. Then it wouldn't be my problem any more. > > > > Maybe I could do a loud warning > > "Not enabling DISCARD on RAID5 because we cannot trust committees. > > Set "md_mod.willing_to_risk_discard=Y" if your devices reads discarded > > sectors as zeros" > > > > and add an appropriate module parameter...... > > I had the same thought and would be happy with this too. I was going to > update Heinz's patch to have the same default off but allow user to > enable: > https://git.kernel.org/cgit/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=for-next&id=8e0cff64f35971135a6de7907bbc8c3a010aff8f > > But I'd love to just follow what you arrive at with MD (using the same > name for the module param in dm-raid). > > I'm open to getting this done now and included in 3.18 if you are. > > Mike How about something like this? I want to keep it well away from regular API stuff as I hope it is just a temporary hack until a more general solution can be found and implemented. Thanks, NeilBrown diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c index 183588b11fc1..3ed668c5378c 100644 --- a/drivers/md/raid5.c +++ b/drivers/md/raid5.c @@ -64,6 +64,10 @@ #define cpu_to_group(cpu) cpu_to_node(cpu) #define ANY_GROUP NUMA_NO_NODE +static bool devices_handle_discard_safely = false; +module_param(devices_handle_discard_safely, bool, false); +MODULE_PARM_DESC(devices_handle_discard_safely, + "Set to Y if all devices in array reliably return zeroes on reads from discarded regions"); static struct workqueue_struct *raid5_wq; /* * Stripe cache @@ -6208,7 +6212,7 @@ static int run(struct mddev *mddev) mddev->queue->limits.discard_granularity = stripe; /* * unaligned part of discard request will be ignored, so can't - * guarantee discard_zerors_data + * guarantee discard_zeroes_data */ mddev->queue->limits.discard_zeroes_data = 0; @@ -6233,6 +6237,18 @@ static int run(struct mddev *mddev) !bdev_get_queue(rdev->bdev)-> limits.discard_zeroes_data) discard_supported = false; + /* Unfortunately, discard_zeroes_data is not currently + * a guarantee - just a hint. So we only allow DISCARD + * if the sysadmin has confirmed that only safe devices + * are in use but setting a module parameter. + */ + if (!devices_handle_discard_safely) { + if (discard_supported) { + pr_info("md/raid456: discard support disabled due to uncertainty.\n"); + pr_info("Set raid456.devices_handle_discard_safely=Y to override.\n"); + } + discard_supported = false; + } } if (discard_supported &&
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