SSD Cache = slower transfer speed?

[SteveJY]

I'm currently only using 3 bays in my F5-422 NAS (3 x 12Tb WD in RAID5) so put in a spare 240Gb SanDisk Plus SSD and enabled SSD cache.

I was a bit surprised when I tried transferring some large files from the NAS and saw the speed drop to around 50Mb/s, rather than the 250-300Mb/s I was getting before adding the SSD.

I tried the performance benchmark NASTester 1.7 to see what results it gave with both configurations.

No SSD Cache:

Running a 4000MB file write on T: 5 times...
Iteration 1: 318.64 MB/sec
Iteration 2: 271.88 MB/sec
Iteration 3: 301.75 MB/sec
Iteration 4: 282.24 MB/sec
Iteration 5: 300.44 MB/sec
-----------------------------
Average (W): 294.99 MB/sec
-----------------------------
Running a 4000MB file read on T: 5 times...
Iteration 1: 274.67 MB/sec
Iteration 2: 279.06 MB/sec
Iteration 3: 297.87 MB/sec
Iteration 4: 307.09 MB/sec
Iteration 5: 299.24 MB/sec
-----------------------------
Average (R): 291.59 MB/sec
-----------------------------

SSD Cache Enabled:

Running a 4000MB file write on T: 5 times...
Iteration 1: 191.23 MB/sec
Iteration 2: 196.62 MB/sec
Iteration 3: 217.99 MB/sec
Iteration 4: 210.89 MB/sec
Iteration 5: 225.86 MB/sec
-----------------------------
Average (W): 208.52 MB/sec
-----------------------------
Running a 4000MB file read on T: 5 times...
Iteration 1: 57.71 MB/sec
Iteration 2: 51.68 MB/sec
Iteration 3: 51.99 MB/sec
Iteration 4: 49.62 MB/sec
Iteration 5: 47.67 MB/sec
-----------------------------
Average (R): 51.73 MB/sec
-----------------------------

That's a massive downgrade, especially when it comes to read speed. It confirms the speed difference I was seeing in the real world transfers between my NAS & PC. Surely that can't be normal behaviour?

Has anyone else seen this issue, or is it a problem with my specific SSD/configuration?

[TMSupport]

We will test here first and get back to you later.

[SteveJY]

I'd be interested to see the results of your testing.

In the meantime, I've carried out the same test with a 500Gb Samsung 860 EVO SSD as cache:

Running a 4000MB file write on T: 5 times...
Iteration 1: 262.54 MB/sec
Iteration 2: 229.94 MB/sec
Iteration 3: 253.35 MB/sec
Iteration 4: 261.47 MB/sec
Iteration 5: 261.65 MB/sec
-----------------------------
Average (W): 253.79 MB/sec
-----------------------------
Running a 4000MB file read on T: 5 times...
Iteration 1: 201.59 MB/sec
Iteration 2: 208.67 MB/sec
Iteration 3: 206.16 MB/sec
Iteration 4: 214.27 MB/sec
Iteration 5: 201.04 MB/sec
-----------------------------
Average (R): 206.35 MB/sec
-----------------------------

I tested again before adding the SSD cache and got read/write results of around 290Mb/s as I did previously.

The results are a lot better using the Samsung 860 EVO rather than the SanDisk Plus, but even with that newer SSD there still seems to be a transfer speed penalty when cache is enabled.

[TMroy]

Your testing result match with our testing. As the SSD cache mode set as default in TOS is "write through" mode, with this mode, you will get better security on your data, but you will experience latency as you have to write to two places every time.
https://shahriar.svbtle.com/Understandi ... ith-python

In near future, we will update the SSD cache feature to allow users set the cache mode as they need.

[SteveJY]

Thanks for the information. The impact on write speed makes sense, but I don't really understand why it would slow down read speed so significantly?

During the <1 hour while the Samsung 860 SSD was enabled as cache, I also experience 2 crashes while writing files to the NAS. Connection to the NAS was suddenly lost and all flashing of activity lights stopped. There was no reply to ping and it could no longer be found in TNAS on my PC. I had to hold down the power button for a hard reset to get it working again. There wasn't anything in the system log when I logged back in to TOS.

After disabling cache I've been copying files backwards and forwards for a couple of hours without any further stability issues. I've tested the SSD separately and it doesn't seem to have any issues.

For now I'll definitely stick with the HDDs and not bother with SSD cache.

[TMroy]

Well, the reading speed from your Samsung 860 SSD was fine and which shows normal compare to our records. BUT reading speed from the sandisk SSD was not like such case, I afraid of that may be a compatibility issue.

Regrading the crash issue, we did not experience the same from our SSD cache testing, but we will mark down your case and verify it in our future testing.

[SteveJY]

It looks like the crash issue wasn't anything to do with the SSD. I had a couple more crashes after the SSD was no longer enabled. I'll make another post to try to deal with that topic.

[TMroy]

It looks like the crash issue wasn't anything to do with the SSD. I had a couple more crashes after the SSD was no longer enabled. I'll make another post to try to deal with that topic.

I afraid of that something wrong with your drives.
please remove all your drives, installed with only one SSD, and installed a fresh new TOS. check if the system can work well with SSD.

[SteveJY]

What I've tried so far: I removed all the drives and followed the testing with WD diagnostic software suggested in this FAQ - all 4 drives have passed the extended test without any errors.

I started from scratch again with those drives, deleted everything, reformatted the RAID, installed the latest TOS, then tried to re-transfer my data from the backup. It ran for a few hours, but ultimately crashed again in the middle of the transfer.

While doing that the 500Gb Samsung 860 SSD had been left in the 5th bay. I'd disabled SSD cache on the volume, so didn't think about actually pulling the drive. Is there any possibility that the SSD could be causing the crashes even if it isn't actually in use?

I'm now trying to follow your advice here - I pulled the 4 HDDs again and started from scratch with just the SSD left in. Online installation of TOS and formatting of the drive seemed to work correctly. I'll try running that for a while to see if I get a crash.

If it doesn't crash with just the SSD then I'm not sure how to proceed without having to replace all 4 of my HDDs?

[TMroy]

if system works well with only SSD installed, then format your all drive on computer and try again.