Re: [RFC PATCH 1/1] dm: add clone target

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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




[Index of Archives]     [DM Crypt]     [Fedora Desktop]     [ATA RAID]     [Fedora Marketing]     [Fedora Packaging]     [Fedora SELinux]     [Yosemite Discussion]     [KDE Users]     [Fedora Docs]

  Powered by Linux