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
>
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