Re: [PATCH 2/2] md/bitmap: Add chunk-count-based bitmap flushing

From: Jonathan Derrick
Date: Thu Oct 13 2022 - 18:19:14 EST

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On 10/10/2022 12:18 PM, Song Liu wrote:
> On Fri, Oct 7, 2022 at 11:58 AM Jonathan Derrick
> <jonathan.derrick@xxxxxxxxx> wrote:
>>
>>
>>
>> On 10/7/2022 11:50 AM, Song Liu wrote:
>>> On Thu, Oct 6, 2022 at 3:09 PM Jonathan Derrick
>>> <jonathan.derrick@xxxxxxxxx> wrote:
>>>
>>> [...]
>>>
>>>> diff --git a/drivers/md/md-bitmap.h b/drivers/md/md-bitmap.h
>>>> index cfd7395de8fd..e0aeedbdde17 100644
>>>> --- a/drivers/md/md-bitmap.h
>>>> +++ b/drivers/md/md-bitmap.h
>>>> @@ -11,10 +11,12 @@
>>>> /* version 4 insists the bitmap is in little-endian order
>>>> * with version 3, it is host-endian which is non-portable
>>>> * Version 5 is currently set only for clustered devices
>>>> ++ * Version 6 supports the flush-chunks threshold
>>>> */
>>>> #define BITMAP_MAJOR_HI 4
>>>> #define BITMAP_MAJOR_CLUSTERED 5
>>>> #define BITMAP_MAJOR_HOSTENDIAN 3
>>>> +#define BITMAP_MAJOR_CHUNKFLUSH 6
>>>>
>>>> /*
>>>> * in-memory bitmap:
>>>> @@ -135,7 +137,8 @@ typedef struct bitmap_super_s {
>>>> * reserved for the bitmap. */
>>>> __le32 nodes; /* 68 the maximum number of nodes in cluster. */
>>>> __u8 cluster_name[64]; /* 72 cluster name to which this md belongs */
>>>> - __u8 pad[256 - 136]; /* set to zero */
>>>> + __le32 daemon_flush_chunks; /* 136 dirty chunks between flushes */
>>>> + __u8 pad[256 - 140]; /* set to zero */
>>>> } bitmap_super_t;
>>>
>>> Do we really need this to be persistent? How about we configure it at run
>>> time via a sysfs file?
>>>
>>> Also, please share more data on the performance benefit of the set.
>>>
>>> Thanks,
>>> Song
>>>
>> Hi Song,
>>
>> Patch 1/2 changes default behavior, which patch 2/2 tries to address.
>
> Have you tried to evaluate the impact on the accuracy of the bitmap?
> Specifically, if we power off the system during writes, do we see data
> or parity mismatch that is not covered by the bitmap?
Fair. I'm assuming this has to do with md_bitmap_init_from_disk()'s
outofdate BITMAP_STALE check? And my patch 1/2 would likely guarantee
a full resync unless the system was lost just after during the daemon
wake time. However patch 2/2 increases the likelihood of reading a good
bitmap.

>
>> I can change it to be configurable via sysfs instead.
>> Should there be a default?
>
> If there is any impact on bitmap accuracy. I think the default should
> work identical as before the set. IOW, we should not delay the bitmap
> update.
With results like mine, I'm under the impression bitmap=internal is not
regularly used for write-heavy workloads [1].

The thing is, is that it's not very consistent right now. I've had runs
where the bitmap isn't updated for minutes until the run ends, and then
I have most runs where it's doing it every other I/O or so. And it seems
to depend on the number of chunks relative to the device size (if it can
fit in a single page.)

I have v2 coming which should help fix a few of these inconsistencies.

[1] Similar results https://blog.liw.fi/posts/write-intent-bitmaps/

>
> Thanks,
> Song
>
>>
>>
>> Here are my observations via biosnoop and RAID1, 4M chunksize, 238436 chunks, bitmap=internal
>> fio --name=test --direct=1 --filename=/dev/md0 --rw=randwrite --runtime=60
>> --percentile_list=1.0:25.0:50.0:75.0:90.0:95.0:99.0:99.9:99.99:99..999999:100.0
>>
>>
>> Default, bitmap updates happened concurrently with I/O:
>> bw ( KiB/s): min=18690, max=30618, per=99.94%, avg=23822.07, stdev=2522.73, samples=119
>> iops : min= 4672, max= 7654, avg=5955.20, stdev=630.71, samples=119
>>
>> TIME(s) COMM PID DISK T SECTOR BYTES LAT(ms)
>> 38.090366 md0_raid1 4800 nvme6n1 W 40 4096 0.01
>> 38.090423 md0_raid1 4800 nvme3n1 W 40 4096 0.07
>> 38.090442 md0_raid1 4800 nvme3n1 W 1016633184 4096 0.01
>> 38.090439 md0_raid1 4800 nvme6n1 W 1016633184 4096 0.01
>> 38.090479 md0_raid1 4800 nvme6n1 W 56 4096 0.01
>> 38.090493 md0_raid1 4800 nvme6n1 W 1449894256 4096 0.01
>> 38.090477 md0_raid1 4800 nvme3n1 W 56 4096 0.01
>> 38.090496 md0_raid1 4800 nvme3n1 W 1449894256 4096 0.01
>> 38.090530 md0_raid1 4800 nvme3n1 W 16 4096 0.01
>> 38.090555 md0_raid1 4800 nvme3n1 W 110493568 4096 0.01
>> 38.090538 md0_raid1 4800 nvme6n1 W 16 4096 0.01
>> 38.090551 md0_raid1 4800 nvme6n1 W 110493568 4096 0.01
>> 38.090596 md0_raid1 4800 nvme6n1 W 56 4096 0.01
>> 38.090647 md0_raid1 4800 nvme3n1 W 56 4096 0.06
>> 38.090666 md0_raid1 4800 nvme3n1 W 1455846976 4096 0.01
>> 38.090663 md0_raid1 4800 nvme6n1 W 1455846976 4096 0.01
>> 38.090707 md0_raid1 4800 nvme6n1 W 64 4096 0.01
>> 38.090699 md0_raid1 4800 nvme3n1 W 64 4096 0.01
>> 38.090723 md0_raid1 4800 nvme3n1 W 1665013728 4096 0.01
>> 38.090720 md0_raid1 4800 nvme6n1 W 1665013728 4096 0.01
>> 38.090764 md0_raid1 4800 nvme6n1 W 64 4096 0.01
>> 38.090812 md0_raid1 4800 nvme3n1 W 64 4096 0.06
>> 38.090832 md0_raid1 4800 nvme3n1 W 1637994296 4096 0.01
>> 38.090828 md0_raid1 4800 nvme6n1 W 1637994296 4096 0.01
>>
>>
>>
>>
>> With patch 1/2, bitmaps only update on the 'delay' parameter (default 5s):
>> bw ( KiB/s): min=135712, max=230938, per=100.00%, avg=209308.56, stdev=29254.31, samples=119
>> iops : min=33928, max=57734, avg=52326.78, stdev=7313.57, samples=119
>>
>> TIME(s) COMM PID DISK T SECTOR BYTES LAT(ms)
>> 16.292235 md0_raid1 4841 nvme6n1 W 297367432 4096 0.01
>> 16.292258 md0_raid1 4841 nvme6n1 W 16 4096 0.01
>> 16.292266 md0_raid1 4841 nvme6n1 W 24 4096 0.01
>> 16.292277 md0_raid1 4841 nvme6n1 W 32 4096 0.01
>> 16.292259 md0_raid1 4841 nvme3n1 W 16 4096 0.01
>> 16.292280 md0_raid1 4841 nvme3n1 W 32 4096 0.01
>> 16.292305 md0_raid1 4841 nvme3n1 W 56 4096 0.01
>> 16.292286 md0_raid1 4841 nvme6n1 W 40 4096 0.01
>> 16.292295 md0_raid1 4841 nvme6n1 W 48 4096 0.01
>> 16.292326 md0_raid1 4841 nvme3n1 W 72 1536 0.01
>> 16.292323 md0_raid1 4841 nvme6n1 W 64 4096 0.02
>> 16.292326 md0_raid1 4841 nvme6n1 W 56 4096 0.03
>> 16.292334 md0_raid1 4841 nvme6n1 W 72 1536 0.02
>> 16.300697 md0_raid1 4841 nvme3n1 W 1297533744 4096 0.01
>> 16.300702 md0_raid1 4841 nvme6n1 W 1297533744 4096 0.01
>> 16.300803 md0_raid1 4841 nvme6n1 W 1649080856 4096 0.01
>> 16.300798 md0_raid1 4841 nvme3n1 W 1649080856 4096 0.01
>> 16.300823 md0_raid1 4841 nvme3n1 W 1539317792 4096 0.01
>> 16.300845 md0_raid1 4841 nvme3n1 W 1634570232 4096 0.01
>> 16.300867 md0_raid1 4841 nvme3n1 W 579232208 4096 0.01
>> 16.300889 md0_raid1 4841 nvme3n1 W 1818140424 4096 0.01
>> 16.300922 md0_raid1 4841 nvme3n1 W 412971920 4096 0.02
>> ...
>> 21.293225 md0_raid1 4841 nvme3n1 W 1279122360 4096 0.01
>> 21.293242 md0_raid1 4841 nvme3n1 W 40326272 4096 0.01
>> 21.293223 md0_raid1 4841 nvme6n1 W 1279122360 4096 0.01
>> 21.293243 md0_raid1 4841 nvme6n1 W 40326272 4096 0.01
>> 21.293261 md0_raid1 4841 nvme6n1 W 16 4096 0.01
>> 21.293266 md0_raid1 4841 nvme6n1 W 24 4096 0.01
>> 21.293271 md0_raid1 4841 nvme6n1 W 32 4096 0.01
>> 21.293275 md0_raid1 4841 nvme3n1 W 32 4096 0.01
>> 21.293292 md0_raid1 4841 nvme3n1 W 48 4096 0.01
>> 21.293296 md0_raid1 4841 nvme3n1 W 56 4096 0.01
>> 21.293309 md0_raid1 4841 nvme3n1 W 72 1536 0.01
>> 21.293266 md0_raid1 4841 nvme3n1 W 24 4096 0.01
>> 21.293326 md0_raid1 4841 nvme6n1 W 48 4096 0.05
>> 21.293328 md0_raid1 4841 nvme6n1 W 40 4096 0.06
>> 21.293331 md0_raid1 4841 nvme6n1 W 72 1536 0.03
>> 21.293333 md0_raid1 4841 nvme6n1 W 64 4096 0.04
>> 21.293334 md0_raid1 4841 nvme6n1 W 56 4096 0.05
>> 21.298526 md0_raid1 4841 nvme3n1 W 681973000 4096 0.01
>>
>>
>>
>>
>> Good, but with the granularity of N seconds, it might be too infrequent.
>> Here is chunk-flush=512 (2GB threshold in 4MB chunk size):
>> bw ( KiB/s): min=92692, max=134904, per=100.00%, avg=125127.43, stdev=6758.51, samples=119
>> iops : min=23173, max=33726, avg=31281.55, stdev=1689.63, samples=119
>>
>> TIME(s) COMM PID DISK T SECTOR BYTES LAT(ms)
>> 13.193339 md0_raid1 5972 nvme6n1 W 16 4096 0.01
>> 13.193344 md0_raid1 5972 nvme6n1 W 32 4096 0.01
>> 13.193346 md0_raid1 5972 nvme6n1 W 24 4096 0.01
>> 13.193350 md0_raid1 5972 nvme6n1 W 40 4096 0.01
>> 13.193356 md0_raid1 5972 nvme6n1 W 48 4096 0.01
>> 13.193361 md0_raid1 5972 nvme6n1 W 64 4096 0.01
>> 13.193363 md0_raid1 5972 nvme6n1 W 56 4096 0.01
>> 13.193555 md0_raid1 5972 nvme6n1 W 72 1536 0.20
>> 13.193289 md0_raid1 5972 nvme3n1 W 1912285848 4096 0.01
>> 13.193306 md0_raid1 5972 nvme3n1 W 836455896 4096 0.01
>> 13.193323 md0_raid1 5972 nvme3n1 W 233728136 4096 0.01
>> 13.193339 md0_raid1 5972 nvme3n1 W 16 4096 0.01
>> 13.193344 md0_raid1 5972 nvme3n1 W 24 4096 0.01
>> 13.193362 md0_raid1 5972 nvme3n1 W 48 4096 0.01
>> 13.193365 md0_raid1 5972 nvme3n1 W 64 4096 0.01
>> 13.193366 md0_raid1 5972 nvme3n1 W 56 4096 0.01
>> 13.193574 md0_raid1 5972 nvme3n1 W 72 1536 0.21
>> 13.196759 md0_raid1 5972 nvme3n1 W 89571592 4096 0.01
>> 13.196810 md0_raid1 5972 nvme6n1 W 89571592 4096 0.06
>> 13.196913 md0_raid1 5972 nvme6n1 W 16 4096 0.01
>> 13.196910 md0_raid1 5972 nvme3n1 W 16 4096 0.01
>> 13.199444 md0_raid1 5972 nvme3n1 W 64 4096 0.01
>> 13.199447 md0_raid1 5972 nvme3n1 W 137126232 4096 0.01
>> 13.199515 md0_raid1 5972 nvme6n1 W 137126232 4096 0.08
>> 13.199519 md0_raid1 5972 nvme6n1 W 64 4096 0.08
>> 13.199617 md0_raid1 5972 nvme6n1 W 1216062808 4096 0.01
>> ... (508 ios later)
>> 13.208764 md0_raid1 5972 nvme6n1 W 16 4096 0.01
>> 13.208768 md0_raid1 5972 nvme6n1 W 32 4096 0.01
>> 13.208770 md0_raid1 5972 nvme6n1 W 24 4096 0.01
>> 13.208775 md0_raid1 5972 nvme6n1 W 40 4096 0.01
>> 13.208781 md0_raid1 5972 nvme6n1 W 48 4096 0.01
>> 13.208786 md0_raid1 5972 nvme6n1 W 56 4096 0.01
>> 13.208790 md0_raid1 5972 nvme6n1 W 64 4096 0.01
>> 13.208729 md0_raid1 5972 nvme3n1 W 1607847808 4096 0.01
>> 13.208747 md0_raid1 5972 nvme3n1 W 371214368 4096 0.01
>> 13.208770 md0_raid1 5972 nvme3n1 W 32 4096 0.01
>> 13.208789 md0_raid1 5972 nvme3n1 W 64 4096 0.01
>> 13.208952 md0_raid1 5972 nvme6n1 W 72 1536 0.17
>> 13.209079 md0_raid1 5972 nvme3n1 W 72 1536 0.29
>> 13.212216 md0_raid1 5972 nvme3n1 W 1146106480 4096 0.01
>> 13.212269 md0_raid1 5972 nvme6n1 W 1146106480 4096 0.06
>> 13.212368 md0_raid1 5972 nvme6n1 W 16 4096 0.01
>> 13.212365 md0_raid1 5972 nvme3n1 W 16 4096 0.01
>>
>>
>> Without 1/2: 6k iops
>> With 1/2: 52k iops
>> With 2/2 params as above: 31k iops
>>
>> The count calculation could use some improvement to close the iops gap to delay-based flushing
>>
>>>>
>>>> /* notes:
>>>> diff --git a/drivers/md/md.h b/drivers/md/md.h
>>>> index b4e2d8b87b61..d25574e46283 100644
>>>> --- a/drivers/md/md.h
>>>> +++ b/drivers/md/md.h
>>>> @@ -497,6 +497,7 @@ struct mddev {
>>>> struct mutex mutex;
>>>> unsigned long chunksize;
>>>> unsigned long daemon_sleep; /* how many jiffies between updates? */
>>>> + unsigned int daemon_flush_chunks; /* how many dirty chunks between updates */
>>>> unsigned long max_write_behind; /* write-behind mode */
>>>> int external;
>>>> int nodes; /* Maximum number of nodes in the cluster */
>>>> --
>>>> 2.31.1
>>>>

Next message: Colin Ian King: "[PATCH] parisc: Fix spelling mistake "mis-match" -> "mismatch""
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In reply to: Song Liu: "Re: [PATCH 2/2] md/bitmap: Add chunk-count-based bitmap flushing"
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