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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? Heinz On 09/24/2014 06:21 AM, NeilBrown
wrote:
On Tue, 23 Sep 2014 23:05:51 -0500 Brassow Jonathan <jbrassow@xxxxxxxxxx> wrote:On Sep 23, 2014, at 9:20 PM, Martin K. Petersen wrote:"Neil" == NeilBrown <neilb@xxxxxxx> writes:Neil, Several people have pointed out the issues with the reliability of discard_zeroes_data in the past. My objection with Heinz' patch is purely that we are perpetuating an interface that we have come to learn that we can't entirely trust. Neil> If it is a "hint", then why isn't it Neil> "discard_sometimes_zeroes_data"? Because at the time we put it in it was expected to be a solid guarantee. However, as it turns out the spec is vaguely worded and only says that the device must return zeroes if a block has been successfully discarded. But since discard itself is only a hint the device is free to silently ignore all or parts of the request. *sigh* There were several approaches being entertained in both T10 and T13 about how to tighten this up, including a FORCE bit that would cause the device to return an error if parts of the request were being ignored. I was hoping that we would be able to use that. However, the consensus at the T10 meeting this summer was that WRITE SAME is the only reliable way to ensure that zeroed blocks are returned. And the spec was amended to clarify that. Neil> If a read from a discarded region doesn't reliably return zeros, Neil> then raid5 cannot support discard. Should I turn of discard Neil> support in raid5? I've been on the fence about this for a while. The good news is that while the spec has been vague and many battles fought in the standards bodies, most vendors are reasonably sane when it comes to implementing this. We haven't had any corruption horror stories from the field that I'm aware of. The first couple of generations of SSDs were a travesty in this department but current devices behave reasonably well. Hardware RAID vendors generally solve the RAID5 SSD problem by explicitly whitelisting drives that are known to do the right thing. Sadly we don't have a good device list. And for most drives the discards only get dropped in rare corner cases so it's not trivial to write a test tool to generate such a list. I was really hoping we'd get better guarantees from the standards folks this summer. But as the spec is currently written switching to a WRITE SAME bias in our discard heuristics may have the opposite effect of what we desire. That's because the spec mandates that if a device can not discard the blocks in the request it must write zeroes to them. IOW, we get the determinism at the expense of potentially ending up writing the blocks in question, thus increasing write wear. Probably not what we want for the discard use case. I am in the process of tweaking the discard code to adhere to the August 27th T10 SBC-4 draft. The net takeaway is that if you depend on reading zeroes back you should be using blkdev_issue_zeroout() and not blkdev_issue_discard(). raid5.c effectively needs to translate discard requests from above to zeroouts below. I am torn between making blkdev_issue_zeroout() unprovision if possible or having an explicit allocate flag. Some users only care about getting zeroes back but others use blkdev_issue_zeroout() to preallocate blocks (which is the current behavior). Right now we have: discard Unprovision, may or may not return zeroes zeroout Provision, return zeroes We'll have to move to a model where we have a third variant: discard Unprovision, may or may not return zeroes zeroout, deallocate Unprovision if possible, return zeroes zeroout, allocate Provision, return zeroes The zeroout, deallocate variant would be enabled if the device supports WRITE SAME w/UNMAP and has the LBPRZ flag set (and is not our libata SATL). Discard is the bane of my existence :(Thanks for the detailed explanation Martin!! It seems that "discard" still comes with a strong "Caveat Emptor" Sad.It's not really so bad if it doesn't return zero's, is it? As long as it returns the previous contents. You mentioned that the discard was a hint - the drive may or may not discard the entire portion. If that means the portions that are discarded will always return zero and the portions that aren't will always return their previous contents, is there any problem? The real issue is randomness, no? Could this be why "we haven't had any corruption horror stories", or is it because we got lucky with sane vendors?For RAID5 it is not acceptable to return "zeros or the original content". This is because of the possibility of RMW cycles to update the parity block. As "discard" the doesn't treat all blocks in a stripe in an identical way will cause the parity block to become incorrect, and it will stay incorrect through multiple RMW cycles until an RCW corrects it. A device failure while the parity block is incorrect will lead to data corruption. The only real solution I can think of is to maintain an 'allocated' bitmap and clear bits when DISCARDing. When a write happens to a location that is not 'allocated' the whole bit worth gets resynced (or maybe zeroed-out). Grumble. Thanks again, NeilBrown -- =============================================================== Heinz Mauelshagen +49 2626 141200 Consulting Development Engineer FAX +49 2626 924446 Red Hat GmbH Am Sonnenhang 11 56242 Marienrachdorf Germany heinzm@xxxxxxxxxx =============================================================== |
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