After upgrading to 5.0, I noticed that my writes speeds (by write, I mean copy from PC to server over ethernet) were really mediocre - around 20 MB/s  (to be honest, they might have been that slow before the upgrade).  Anyway, I thought I would add a cache disk.  I had a 128 MB SSD lying around so I added that and set it as the cache disk.  To my astonishment, the write speed was now around 10 MB/s !  I tried various write experiments and sometime I would get higher speeds like 45 MB/s, but mostly just 10.  By that I mean, once or twice I got a high speed and all the rest of time it is pretty consist between 12-8 MB/s.

 

OK, I thought, maybe its the SATA port.  I had it on a PCI HBA (see my sig for model) so I moved it to a MB port.  No change.  I then added an extra 2 TB HDD I had and made it the cache.  No change!

 

The really weird thing is that when I write directly to a disk (over the 1 gig ethernet) but bypassing the cache (e.g. copying directly to \\Tower\disk9), I get the usual 20 MB/s write speed.

 

Through all this, read speed is consistently between 120-90 MB/s.

 

I then ran some tests:

 

Transfers from PC to Server

(disk9 is a WD30EZRX)

 

1) PC -> \\Videos  (this is my top level share)                    10 MB/s  occasionally, 45 MB/s on Supermicro HBA

2) PC -> \\Tower\disk9 (to test bypassing the cache)          22 MB/s

3) PC -> \\Tower\cache (to test writing to cache directly)      30 MB/s

 

Transfer from Server to PC (reads)

4) \\Tower\disk9 -> PC                                                    104 MB/s  !!!

 

I then did some copies "locally" by telneting to the server and using the Linux cp to copy files from place to place.

 

5) /mnt/disk9  -> /mnt/cache                                            116 MB/s (this is when I had the SSD as the cache)

6) /mnt/disk9  -> /mnt/disk4                                              36 MB/s (HDD to HDD)

 

So here are my conclusions:

1.  SATA ports, drivers, etc are OK (since disk to disk local is fast)

2. Ethernet is OK (since I get 100+ MB/s read over ethernet)

3. Enabling the cache is actually what is causing the slowdown since, when I disable the cache, I revert to the 20 MB/s read (very consistent).

 

So, anybody got any ideas?  Any suggestion as to what I might try next to isolate the cause?

 

Thanks.

 

Slower-than-expected write speeds are often caused by a discontinuity in one of the wires or connector pins in an Ethernet cable ... although that almost always slows it down to 100Mb speeds (~ 10MB/s) ... not to the 20MB you're seeing.    Nevertheless, I'd try a different set of cables (be sure to change EVERY cable involved in the transfer ... PC to router; router to UnRAID; UPS to UnRAID if you have one).

 

Note that is it NOT unusual for this to only impact one direction of the transfers (it depends on which specific cable/connection has the discontinuity).

 

Try that and see if it makes a difference.  Then we can look at other possibilities.

 

 

Slower-than-expected write speeds are often caused by a discontinuity in one of the wires or connector pins in an Ethernet cable ... although that almost always slows it down to 100Mb speeds (~ 10MB/s) ... not to the 20MB you're seeing.    Nevertheless, I'd try a different set of cables (be sure to change EVERY cable involved in the transfer ... PC to router; router to UnRAID; UPS to UnRAID if you have one).

 

Note that is it NOT unusual for this to only impact one direction of the transfers (it depends on which specific cable/connection has the discontinuity).

 

Try that and see if it makes a difference.  Then we can look at other possibilities.

 

Damn! You're good!  I have a total of 5 cables and 4 switches between the server and my test PC.  I replaced the cable between the server and the closest switch and I now get the following results:

 

PC -> \\Tower\cache                110-120 MB/s

PC -> \\Videos (share)              50-60 MB/s

PC -> \\Tower\disk9                  30 MB/s

 

I knew that RX and TX were full duplex on ethernet, I just did not connect the read/write mismatch.  BTW, I check the cable on a continuity tester and it tested good

 

So, now the question is:  why is a direct write to the cache disk so much faster than a "cached" write to the Videos share?  I would have thought that, since the write to the array with the cache enabled does not compute parity, that it would be the same speed as a "direct" write.  Any ideas on that?

 

 

Glad it's resolved.

 

I agree it's strange that direct writes to the cache are so much quicker than writes to a cached share.  Clearly they're writing to the same disk!!

 

Just to be sure this isn't due to either the READ speed of what you were writing, or the size of the files,  try it again being sure you write the same file to both the cache disk and the cached share; write it from the same source disk on your test PC; and reboot your test PC between the writes to ensure the file isn't still buffered in RAM after the first write.

 

I have seen similiar behavior. Faster directly to cache drive (doubled) vs to a cache share.

I've also seen the same behavior.  A write directly to my cache drive is ~80MB/s, the same write to a cached share is ~40MB/s.  I can write to the protected array at ~40MB/s, so using cached shares is effectively useless for me.

 

I know this is a stretch, but maybe the overhead of user shares has something to do with it?

 

 

Write to: \\tower\cache\share

 

The mover will delete the cache\share directory.

Yes we know that, but we wonder why we have to do that. Should be able to save just on the \\tower\share as it puts it on the cache either way.

 

 

Write to: \\tower\cache\share

 

The mover will delete the cache\share directory.

Any ideas if this can be fixed? i get 105MB/s on cache but on cached shares i get same speed as non cached shares 60MB/s

Are you certain that Use Cache is selected for the share?

Are you certain that Use Cache is selected for the share?

 

yes i can see the file uploading into the cache dir under the correct share folder that will be moved later.

like

uploading to movies share the file goes to /cache/movies/file at 60MB/s

uploading directly to /cache  at 105MB/s

 

Now i notice why extracting with sabnzbd was slower on my cache drive I set downloads share to be cached and when i set sabnzbd to extract here its slower then extracting directly to cache

if its set to /mnt/user/Downloads/ its maxed out at 60MB/s rar extracting

vs /mnt/cache

Attach a syslog and SMART reports for the drive.