Hey folks,

 

I have a little bit of a strange issue with my SSD Cache. I am using two 256 GB SSDs in my system alongside two 6TB WD RED HDDs. I have a media SMB share that I use for my movie and TV show collection. It is set to "Yes" on the 'use cache pool' setting. 

 

My problem is, that when I upload to my array, the upload speed is pretty good right out of the gate. I max out my network connection when doing so, uploading right around 100MB/s. Then, as the transfer proceeds, I lose that speed. I have attached a graph of my transfer speeds, the big hump in the middle is immediately after I 'unpaused' the file transfer. That hump is where my speeds averaged around 100MB/s. After then, you can see it dropped significantly. After this point, I do not see the speeds return. And the files are video files, so it all should be sequential?

 

Thank you for any insight.

That big peak is probably the filling of the RAM cache.  (Unused RAM used by Unraid to buffer disk writes.)  Once that fills up the process slows to that the physical disks + OS file creation overhead permit.

 

I have observed that with vary large files (say, bluRay iso's) , the write speed to my SSD will be very close to 100MB/s.  When I am doing a data backup of thousands of small files, the speed will drop back like you are finding.  Unraid appears to have a very large file creation overhead  when writing to a User Share.

5 hours ago, Frank1940 said:

Once that fills up the process slows to that the physical disks + OS file creation overhead permit.

So my SSD can only truly write at 10MB/s? That sounds crazy low. If that's the case, should I look into replacing it?
 

5 hours ago, Frank1940 said:

I have observed that with vary large files (say, bluRay iso's) , the write speed to my SSD will be very close to 100MB/s. 

That is what I would expect. I was copying MKV files (so rather large) and I figured those would be sequential and wouldn't take up too much random I/O.

sadly you did not mention WHAT kind of SSD you are using for the cache.

There is a big difference in performance between SATA and NVMe types, so we cannot see what expectations we should have to your transfer speed.

 

Also, if you are using mirrored caches you can expect to cut the max speed by half because every write has to be done twice.

 

But then, all of them should be capable to accept slow 100Mb/s.

 

The bad thing with SSDs is that the ads only tell you about the "maximum transfer speed", not the "sustained transfer speed". So it is very common that a drive has a very small cache only, transfers start fast and after a certain amount of data it drops to "slow as a dog" or even stalls completely. This is normal, because writing to flash is not really fast. But the companies add a lot of wisdom to compensate this. You can easily test this out by transfering a large file and watch the drop point. Then you can decide if this is big enough for your demands, or if you have to buy a different drive.

It took me quite long to get an NVMe drive that can sustain 1Gb/s for at least 100Gb of data. And yes, those are not the cheapest ones too. I've tried 5 different one before I was satisfied, and, yes, even within the same brands small changes in names (like -EVO, -PRO) can make a huge difference)

 

But back to your lousy 8Mb/s... thats ridiculous even for a cheap drive. You should start by disabling the mirror (taking out the 2nd ssd from the cache pool) and see if it increases at least to 16Mb/s (which would still be much too low).

 

 

 

8 hours ago, rjdipcord said:

So my SSD can only truly write at 10MB/s? That sounds crazy low. If that's the case, should I look into replacing it?
 

 

6 hours ago, Michael Meiszl said:

But then, all of them should be capable to accept slow 100Mb/s

 

Be careful with nomenclature.   MB = Megabytes    Mb = Megabits.    Windows generally reports MB's  and Unraid generally uses Mb's.  It is a big confusion factor for many folks. 

 

(Plus, there is the    1000KB = 1MB    and    1024KB = 1MB     issue to add even more confusion.  This one -- in the scientific/math communities -- has specific nomenclature to distinction which is being use but outside of those communities, it is seldom used.

 

And here is the two of them mixed!!!)  

 

Another thing, a 100MB/s is close to the limit for Gb LAN connections and that LAN connection is generally the bottleneck in transfer speed for most folks.  Most HD's and virtually all SSD's in their raw read/write capability should be able to saturate Gb LAN.  The problem usually comes in the file overhead on both ends of the connection--  Particularity when HD's are involved.  (Track-to-track head movement and rotational latency to position the head over the actual data.  Remember that for every file is that is transferred, the OS's on both ends has to retrieve/update the file structure data.)

 

I have not had a problem with my Samsung 870EVO 500GB SSD (simple one drive cache setup) being able to sustain 100MB/s on file transfers of 30+ GB mkv/iso files but the transfer of smaller files during the backup of my Windows computers will have a max speed (as reported by Windows Explorer) will be under 50MB/s after the initial loading of the RAM buffer*.  And, in certain directories, well under that speed.  I have become philosophical about this situation.  So it takes eighteen minutes rather than ten minutes.  It is also well to realize that increases in transfer speeds is matter of diminishing returns--  Doubling the speed of a 20 minute transfer saves  10 minutes but doubling that transfer speed only saves 5 minutes, and so on...

==========================

* And Unraid does use a RAM buffer as all drive writes are queued and done at the convenience of the OS when CPU cycles are available to handle the load.  The actual Max delay time before a write to the drive is an adjustable parameter as a power loss can cause data loss if the failure happens between when the write data is generated and actual file write to HD is finished.

 

I can remember when MS added this feature to Windows for Workgroups (as I recall).  Prior to that time, the computer was simply unavailable to do anything else while a drive write was going on.  Since Windows for Workgroups was a primitive multitasking system, if one were typing and a background save would be required, the characters stopped appearing on the screen.  When the write finished, they would suddenly just popup on the screen.  (Oh, many folks had those background saves setup for intervals like ten minutes as system crashes could happened several times a day...)

Edited April 18, 20224 yr by Frank1940