On 7/30/19 12:20 AM, Heinz Mauelshagen wrote: > Hi Nikos, > > thanks for providing these benchmarks which seem to confirm the > advantages of clone vs. a snapshot/raid1 stack. > > Can you please provide 'dmsetup table' for both configurations for > completeness? > > Heinz > Hi Heinz, Yes, of course. The below 'dmsetup table' output is for the 4K region/chunk size benchmark. The 'dmsetup table' output for the rest of the benchmarks is the same, changing only the region/chunk sizes of dm-clone and dm-snapshot. dm-clone stack (dmsetup table) ============================== source--vg-origin--lv: 0 629145600 linear 8:16 2048 dest--vg-meta--lv: 0 65536 linear 259:0 629147648 clone: 0 629145600 clone 254:1 254:0 254:2 8 dest--vg-clone--lv: 0 629145600 linear 259:0 2048 dm-snapshot + dm-raid stack (dmsetup table) =========================================== mirrorvg-snap-cow: 0 104857600 linear 259:0 629155840 mirrorvg-raid1--lv_rimage_1: 0 629145600 linear 259:0 10240 mirrorvg-snap: 0 629145600 snapshot 254:5 254:6 P 8 mirrorvg-raid1--lv_rimage_0: 0 629145600 linear 8:16 10240 mirrorvg-raid1--lv-real: 0 629145600 raid raid1 3 0 region_size 1024 2 254:0 254:1 254:2 254:3 mirrorvg-raid1--lv: 0 629145600 snapshot-origin 254:5 mirrorvg-raid1--lv_rmeta_1: 0 8192 linear 259:0 2048 mirrorvg-raid1--lv_rmeta_0: 0 8192 linear 8:16 2048 Nikos > On 7/22/19 10:16 PM, Nikos Tsironis wrote: >> On 7/17/19 5:41 PM, Heinz Mauelshagen wrote: >>> Hi Nikos, >>> >>> thanks for elaborating on those details. >>> >>> Hash table collisions, exception store entry commit overhead, >>> SSD cache flush issues etc. are all valid points relative to performance >>> and work set footprints in general. >>> >>> Do you have any performance numbers for your solution vs. >>> a snapshot one showing the approach is actually superior in >>> in real configurations? >> Hi Heinz, >> >> Please see below for detailed benchmark results. >> >>> I'm asking this particularly in the context of your remark >>> >>> "A write to a not yet hydrated region will be delayed until the >>> corresponding >>> region has been hydrated and the hydration of the region starts >>> immediately." >>> >>> which'll cause a potentially large working set of delayed writes unless >>> those >>> cover the whole eventually larger than 4K region. >>> How does your 'clone' target perform on such heavy write situations? >>> >> This situation occurs only when the writes are smaller than the region >> size of dm-clone. E.g., if the user sets a region size of 64K and issues >> 4K writes. >> >> In this case, we experience a performance drop due to COW. This is true >> _both_ for dm-snapshot and dm-clone and is _unavoidable_. >> >> But, the common case will be setting a region size equal to the file >> system block size, e.g., 4K, and thus avoiding the COW overhead. Note >> that LVM snapshots _already_ use 4K as the _default_ chunk size. >> >> Nevertheless, even for larger region/chunk sizes, dm-clone outperforms >> the dm-snapshot based solution, as is evident by the following >> performance measurements. >> >>> In general, performance and storage footprint test results based on the >>> same set >>> of read/write tests including heavy loads with region size variations >>> run on 'clone' >>> and 'snapshot' would help your point. >>> >>> Heinz >>> >> I used fio to run a series of read and write tests that compare the >> performance of dm-clone against your proposed dm-snapshot over dm-raid >> solution. >> >> I used a 375GB spinning disk as the origin device storing the data to be >> cloned and a 375GB SSD as the clone device and for storing both >> dm-clone's metadata and dm-snapshot's exceptions (COW space). >> >> dm-clone stack (dmsetup ls --tree) >> ================================== >> >> clone (254:3) >> ├─source--vg-origin--lv (254:2) >> │ └─ (8:16) >> ├─dest--vg-clone--lv (254:0) >> │ └─ (259:0) >> └─dest--vg-meta--lv (254:1) >> └─ (259:0) >> >> dm-snapshot + dm-raid stack (dmsetup ls --tree) >> =============================================== >> >> mirrorvg-snap (254:7) >> ├─mirrorvg-snap-cow (254:6) >> │ └─ (259:0) >> └─mirrorvg-raid1--lv-real (254:5) >> ├─mirrorvg-raid1--lv_rimage_1 (254:3) >> │ └─ (259:0) >> ├─mirrorvg-raid1--lv_rmeta_1 (254:2) >> │ └─ (259:0) >> ├─mirrorvg-raid1--lv_rimage_0 (254:1) >> │ └─ (8:16) >> └─mirrorvg-raid1--lv_rmeta_0 (254:0) >> └─ (8:16) >> mirrorvg-raid1--lv (254:4) >> └─mirrorvg-raid1--lv-real (254:5) >> ├─mirrorvg-raid1--lv_rimage_1 (254:3) >> │ └─ (259:0) >> ├─mirrorvg-raid1--lv_rmeta_1 (254:2) >> │ └─ (259:0) >> ├─mirrorvg-raid1--lv_rimage_0 (254:1) >> │ └─ (8:16) >> └─mirrorvg-raid1--lv_rmeta_0 (254:0) >> └─ (8:16) >> >> fio configuration >> ================= >> >> 1. Random Read/Write latency benchmark >> >> ioengine=psync, bs=4K, numjobs=1, direct=1, timeout=90, time_based=1, >> rw=randwrite/randread >> >> 2. Random Read/Write IOPS benchmark >> >> ioengine=libaio, bs=4K, numjobs=1, direct=1, iodepth=32, timeout=90, >> time_based=1, rw=randwrite/randread >> >> 3. Sequential Read/Write Bandwidth >> >> ioengine=libaio, bs=256K, numjobs=1, direct=1, iodepth=32, timeout=90, >> time_based=1, rw=write/read >> >> Baseline >> ======== >> >> As a reference, the benchmark results for the raw devices: >> >> +--------+--------------------+-----------------+--------------+ >> | device | rand-write latency | rand-write IOPS | seq-write BW | >> +--------+--------------------+-----------------+--------------+ >> | HDD | 701 usec | 1425 | 120 MB/s | >> | SSD | 72.6 usec | 64490 | 390 MB/s | >> +--------+--------------------+-----------------+--------------+ >> >> +--------+-------------------+----------------+-------------+ >> | device | rand-read latency | rand-read IOPS | seq-read BW | >> +--------+-------------------+----------------+-------------+ >> | HDD | 1.4 msec | 712 | 120 MB/s | >> | SSD | 122 usec | 150920 | 701 MB/s | >> +--------+-------------------+----------------+-------------+ >> >> dm-clone vs dm-snapshot+dm-raid >> =============================== >> >> Latency benchmark >> ----------------- >> >> 1. Random write latency >> >> +-------------------+-----------+-------------+ >> | region/chunk size | dm-clone | dm-snapshot | >> +-------------------+-----------+-------------+ >> | 4 KB | 75.7 usec | 6.8 msec | >> | 8 KB | 1.9 msec | 17.7 msec | >> | 16 KB | 2.1 msec | 15.8 msec | >> | 32 KB | 2.2 msec | 33.6 msec | >> | 64 KB | 2.6 msec | 31.2 msec | >> | 128 KB | 3.8 msec | 35.7 msec | >> +-------------------+-----------+-------------+ >> >> * dm-snapshot+dm-raid has 7.5 to 90 times _more_ write latency than >> dm-clone. >> >> * For the common case of a 4 KB region/chunk size, dm-clone has minimal >> overhead over the SSD device. >> >> * Even for region/chunk sizes greater than 4KB dm-clone's overhead is >> minimal compared to dm-snapshot+dm-raid. >> >> 2. Random read latency >> >> +-------------------+----------+-------------+ >> | region/chunk size | dm-clone | dm-snapshot | >> +-------------------+----------+-------------+ >> | 4 KB | 1.5 msec | 10.7 msec | >> | 8 KB | 1.5 msec | 9.7 msec | >> | 16 KB | 1.5 msec | 11.9 msec | >> | 32 KB | 1.5 msec | 28.6 msec | >> | 64 KB | 1.5 msec | 27.5 msec | >> | 128 KB | 1.5 msec | 27.3 msec | >> +-------------------+----------+-------------+ >> >> * dm-snapshot+dm-raid has 6.5 to 19 times _more_ read latency than >> dm-clone. >> >> * For all region/chunk sizes dm-clone has minimal overhead over the HDD >> device. >> >> IOPS benchmark >> -------------- >> >> 1. Random write IOPS >> >> +-------------------+----------+-------------+ >> | region/chunk size | dm-clone | dm-snapshot | >> +-------------------+----------+-------------+ >> | 4 KB | 62347 | 3758 | >> | 8 KB | 696 | 388 | >> | 16 KB | 667 | 217 | >> | 32 KB | 614 | 207 | >> | 64 KB | 531 | 186 | >> | 128 KB | 417 | 159 | >> +-------------------+----------+-------------+ >> >> * dm-clone achieves 1.8 to 16.6 times _more_ IOPS than >> dm-snapshot+dm-raid. >> >> * For the common case of a 4 KB region/chunk size, dm-clone has minimal >> overhead over the SSD device. >> >> * Even for region/chunk sizes greater than 4KB dm-clone achieves >> significantly more IOPS than dm-snapshot+dm-raid. >> >> 2. Random read IOPS >> >> +-------------------+----------+-------------+ >> | region/chunk size | dm-clone | dm-snapshot | >> +-------------------+----------+-------------+ >> | 4 KB | 767 | 680 | >> | 8 KB | 714 | 677 | >> | 16 KB | 715 | 338 | >> | 32 KB | 717 | 338 | >> | 64 KB | 720 | 338 | >> | 128 KB | 724 | 339 | >> +-------------------+----------+-------------+ >> >> * dm-clone achieves 1.1 to 2.1 times _more_ IOPS than >> dm-snapshot+dm-raid. >> >> Bandwidth benchmark >> ------------------- >> >> 1. Sequential write BW >> >> +-------------------+------------+-------------+ >> | region/chunk size | dm-clone | dm-snapshot | >> +-------------------+------------+-------------+ >> | 4 KB | 389.4 MB/s | 135.3 MB/s | >> | 8 KB | 390.5 MB/s | 231.7 MB/s | >> | 16 KB | 390.5 MB/s | 213.1 MB/s | >> | 32 KB | 390.4 MB/s | 214.0 MB/s | >> | 64 KB | 390.3 MB/s | 214.0 MB/s | >> | 128 KB | 390.5 MB/s | 211.3 MB/s | >> +-------------------+------------+-------------+ >> >> * dm-clone achieves 1.7 to 2.9 times more write BW than >> dm-snapshot+dm-raid. >> >> * For all region/chunk sizes dm-clone achieves the same write BW as the >> SSD device. >> >> 2. Sequential read BW >> >> +-------------------+------------+-------------+ >> | region/chunk size | dm-clone | dm-snapshot | >> +-------------------+------------+-------------+ >> | 4 KB | 442.8 MB/s | 217.3 MB/s | >> | 8 KB | 443.8 MB/s | 288.8 MB/s | >> | 16 KB | 443.8 MB/s | 275.3 MB/s | >> | 32 KB | 443.8 MB/s | 276.1 MB/s | >> | 64 KB | 443.6 MB/s | 276.1 MB/s | >> | 128 KB | 443.6 MB/s | 275.2 MB/s | >> +-------------------+------------+-------------+ >> >> * dm-clone achieves 1.5 to 2 times more read BW than >> dm-snapshot+dm-raid. >> >> Metadata/Storage overhead >> ========================= >> >> dm-clone had a _maximum_ metadata overhead of around 20 MB for all >> benchmarks. As dm-clone doesn't require any extra COW space for >> temporarily storing the written data (writes just go directly to the >> clone device) this is the _only_ storage overhead incurred by dm-clone, >> irrespective of the amount of the written data >> >> On the other hand, the COW space utilization of dm-snapshot, for the >> bandwidth benchmarks, varied from 11.95 GB to 20.41 GB, depending on the >> amount of written data. >> >> I want to emphasize that after the cloning/syncing is complete we have >> to merge this multi-gigabyte COW space back to the clone/destination >> device. This will cause _further_ performance degradation, which is >> _not_ reflected in the above performance measurements, but _will_ be >> present in real workloads, if the dm-snapshot based solution is used. >> >> >> To summarize, dm-clone performs _significantly_ better than a >> dm-snapshot based solution, on all aspects (latency, IOPS, BW), and with >> a _fraction_ of the storage/metadata overhead. >> >> If you have any more questions, I would be more than happy to discuss >> them with you. >> >> Thanks, >> Nikos >> >>> On 7/10/19 8:45 PM, Nikos Tsironis wrote: >>>> On 7/10/19 12:28 AM, Heinz Mauelshagen wrote: >>>>> Hi Nikos, >>> e> >>>>> what is the crucial factor your target offers vs. resynchronizing such a >>>>> latency distinct >>>>> 2-legged mirror with a read-write snapshot (local, fast exception store) >>>>> on top, tearing the >>>>> mirror down keeping the local leg once fully in sync and merging the >>>>> snapshot back into it? >>>>> >>>>> Heinz >>>>> >>>> Hi Heinz, >>>> >>>> The most significant benefits of dm-clone over the solution you propose >>>> is significantly better performance, no need for extra COW space, no >>>> need to merge back a snapshot, and the ability to skip syncing the >>>> unused space of a file system. >>>> >>>> 1. In order to ensure snapshot consistency, dm-snapshot needs to >>>> commit a completed exception, before signaling the completion of the >>>> write that triggered it to upper layers. >>>> >>>> The persistent exception store commits exceptions every time a >>>> metadata area is filled or when there are no more exceptions >>>> in-flight. For a 4K chunk size we have 256 exceptions per metadata >>>> area, so the best case scenario is one commit per 256 writes. Here I >>>> assume a write with size equal to the chunk size of dm-snapshot, >>>> e.g., 4K, so there is no COW overhead, and that we write to new >>>> chunks, so we need to allocate new exceptions. >>>> >>>> Part of committing the metadata is flushing the cache of the >>>> underlying device, if there is one. We have seen SSDs which can >>>> sustain hundreds of thousands of random write IOPS, but they take up >>>> to 8ms to flush their cache. In such a case, flushing the SSD cache >>>> every few writes significantly degrades performance. >>>> >>>> Moreover, dm-snapshot forces exceptions to complete in the order they >>>> were allocated, to avoid snapshot space leak on crash (commit >>>> 230c83afdd9cd). This inserts further latency in exception completions >>>> and thus user write completions. >>>> >>>> On the other hand, when cloning a device we don't need to be so >>>> strict and can rely on committing the metadata every time a FLUSH or >>>> FUA bio is written, or periodically, like dm-thin and dm-cache do. >>>> >>>> dm-clone does exactly that. When a region/chunk is cloned or >>>> over-written by a write, we just set a bit in the relevant in-core >>>> bitmap. The metadata are committed once every second or when we >>>> receive a FLUSH or FUA bio. >>>> >>>> This improves performance significantly and results in increased IOPS >>>> and reduced latency, especially in cases where flushing the disk >>>> cache is very expensive. >>>> >>>> 2. For large devices, e.g. multi terabyte disks, resynchronizing the >>>> local leg can take a lot of time. If the application running over the >>>> local device is write-heavy, dm-snapshot will end up allocating a >>>> large number of exceptions. This increases the number of hash table >>>> collisions and thus increases the time we need to do a hash table >>>> lookup. >>>> >>>> dm-snapshot needs to look up the exception hash tables in order to >>>> service an I/O, so this increases latency and degrades performance. >>>> >>>> On the other hand, dm-clone is just testing a bit to see if a region >>>> is cloned or not and decides what to do based on that test. >>>> >>>> 3. With dm-clone there is no need to reserve extra COW space for >>>> temporarily storing the written data, while the clone device is >>>> syncing. Nor would one need to worry about monitoring and expanding >>>> the COW device to prevent it from filling up. >>>> >>>> 4. With dm-clone there is no need to merge back potentially several >>>> gigabytes once cloning/syncing completes. We also avoid the relevant >>>> performance degradation incurred by the merging process. Writes just >>>> go directly to the clone device. >>>> >>>> 5. dm-clone implements support for discards, so it can skip >>>> cloning/syncing the relevant regions. In the case of a large block >>>> device which contains a filesystem with empty space, e.g. a 2TB >>>> device containing 500GB of useful data in a filesystem, this can >>>> significantly reduce the time needed to sync/clone. >>>> >>>> This was a rather long email, but I hope it makes the significant >>>> benefits of dm-clone over using dm-snapshot, and our rationale behind >>>> the decision to implement a new target clearer. >>>> >>>> I would be more than happy to continue the conversation and focus on any >>>> other questions you may have. >>>> >>>> Thanks, >>>> Nikos >> -- >> dm-devel mailing list >> dm-devel@xxxxxxxxxx >> https://www.redhat.com/mailman/listinfo/dm-devel > -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
