Fast cache drive vs fast parity drive?

[coldflame]

Hello all,

 

I'm new to Unraid and just set up a prototype server with 3x500 GB 7200 rpm drives. It seems to work fine and I'm moving to building my "production" environment. My goal is to saturate GbE network, i.e. get to approx 100 MB/s writes and reads. Reads were fine but writes were a bit slow, around 35 MB/s with parity. I'd like to get suggestions on how to architect my Unraid server for performance.

 

I plan to use "green" drives for storage but I don't know if I should use a 7200 rpm drive for parity or not. Initially I thought I need a fast parity drive. But then I thought if I have a cache drive then overall performance is bound by the speed of the cache drive. Is that assumption correct? When will the cache drive start writing the data to the data drives? I hope it's after the writes from the clients have finished.

 

Thanks all and I hope to get some good info!

 

[queeg]

Hello all,

 

I plan to use "green" drives for storage but I don't know if I should use a 7200 rpm drive for parity or not. Initially I thought I need a fast parity drive. But then I thought if I have a cache drive then overall performance is bound by the speed of the cache drive. Is that assumption correct? When will the cache drive start writing the data to the data drives? I hope it's after the writes from the clients have finished.

 

Thanks all and I hope to get some good info!

 

 

Optimally, the parity drive should be as fast or faster than the fastest data drive. 

 

You can set the schedule for the cache drive to move it's contents to the array in the middle of the night when noone is using the system if you like.

[Rajahal]

I think your expectations are unrealistic.  Reads are fine, as you said, but the fastest write to an unRAID server ever documented was just a bit over 70 mb/s (and that was using an SSD).

 

A fast parity drive will only help you if you write to multiple drives simultaneously.  If you use a cache drive, it won't make any difference at all.

 

As queeg said, you can schedule the cache drive mover script to run whenever you like, anywhere from once a minute to once a year.  Most of us that use a cache drive leave it on the default setting of once per night at 3:40 am.  The most frequent I would recommend is once per hour - past that, and you may run into issues.  Also, the cache drive mover script will only move files that aren't actively open or being used.

[coldflame]

I think your expectations are unrealistic.  Reads are fine, as you said, but the fastest write to an unRAID server ever documented was just a bit over 70 mb/s (and that was using an SSD).

I agree on expectations - I was coming from the fact that a single 7200 rpm drive can achieve sustained sequential 100 MB/s transfers over GbE from what I've seen and I was hoping that Unraid can provide that level of performance since it does not look it should do anything on reads or cached writes. My WHS box can provide reads at max speed of the actual drive I'm reading from and one of the reasons I'm trying Unraid is because write speeds are inconsistent in WHS.

 

A fast parity drive will only help you if you write to multiple drives simultaneously.  If you use a cache drive, it won't make any difference at all.

This is what I expected - thank you for confirming that!

 

[LVLAaron]

Get the fastest Cache drive you care to afford. Your write speeds should be very fast.

[PeterB]

I agree on expectations - I was coming from the fact that a single 7200 rpm drive can achieve sustained sequential 100 MB/s transfers over GbE from what I've seen...

 

That would be a theoretical maximum, yes.  In order to achieve that in practice, you would need a highly optimised system (network, client and server).

 

... and I was hoping that Unraid can provide that level of performance since it does not look it should do anything on reads or cached writes.

 

I may not have the fastest systems around but, in my experience, 50 MB/s would be a realistic aim - that is the peak I see before system buffers appear to become exhausted.  I'm using an 'older' WD 7200RPM drive for cache, with 64/40MB/s best/worst sequential write performance, according to a benchmark test.  The overall transfer rate on larger files appears to settle around the 40MB/s mark, but remember that a cache drive is nearly always empty, so should be working in the highest performance area.  However, I'm conscious that there appear to be pauses in drive activity during writes (suggesting, to me, that the drive is not the ultimate bottleneck) - perhaps my system could be better configured?

 

Tom has discussed the possibility of improving performance with a rewrite of the disk drivers - however, I'm not clear whether that would affect cache drive performance.

 

My WHS box can provide reads at max speed of the actual drive I'm reading from and one of the reasons I'm trying Unraid is because write speeds are inconsistent in WHS.

 

Do you care to expand on 'inconsistent'?

[lionelhutz]

Cache vs parity is a no brainer if talking write speed to a user share - the cache will easily win.

 

I typically see around 60MBps for both reads and writes with the cache drive.

 

I would not expect to get much higher than this.

 

Peter

 

 

 

[Kaygee]

I agree on expectations - I was coming from the fact that a single 7200 rpm drive can achieve sustained sequential 100 MB/s transfers over GbE from what I've seen and I was hoping that Unraid can provide that level of performance since it does not look it should do anything on reads or cached writes.

 

Reads should be whatever the source drive can deliver unless the array is degraded or busy or your network is slowing it down.

 

A single mechanical drive cant write at a sustained 100MB/s full stop. For some realistic figures look on smallnetbuilders NAS charts. With a good network, good client and good server the fastest NAS boxes hit 70MB/s on sustained writes.

 

A sustained 60MB/s for a 4GB file is realistic for unraid as others have said with a cache drive, the penalty for this is the data is not protected until it is written to the array. For UnRaid array writes 35MB/s is pretty much bang on the money for a mechanical HDD system using parity protection and a couple of drives.

 

 

[Rajahal]

For UnRaid array writes 35MB/s is pretty much bang on the money for a mechanical HDD system using parity protection and a couple of drives.

 

Really?  The max I've seen for sustained writes is around 26 Mb/s.

[bcbgboy13]

For UnRaid array writes 35MB/s is pretty much bang on the money for a mechanical HDD system using parity protection and a couple of drives.

 

Really?  The max I've seen for sustained writes is around 26 Mb/s.

 

Perhaps it is for 7200 rpm parity plus 7200 rpm data drives as coincidentally:

 

7200/5400 is approximately 35/26 

[Rajahal]

That makes sense, as I only ever use green drives.

[Bizarroterl]

I'm using a SSD Cache drive - Intel X25E 64GB.  My write speed varies from 70MB/s to about 95MB/s. GBe, Celeron 420 CPU (1.6Ghz), Intel DQ45EK MB, cheap 4 port PCIe 1x card.  The cache drive is connected to the PCIe card.  Source system is a Win 7 x64 Ult PC w/2.86Ghz CPU with a Intel X25M 80GB SSD(OS) and a 160GB X25M SSD for data (copy from).  Network switch is a Netgear GS108.

 

 

So my vote is get a fast cache drive if you want write speed.  

 

Edit:  Running UnRaid v4.5.6

[Rajahal]

Looks like we have a new record!