SSD recommendation for SLOG device

[Thomymaster]

Hi


I need a recommendation for a SSD i can use as a SLOG device for my FreeNAS box.

I had an ADATA SX900 64GB SSD installed which made my writes (via CIFS) slowed down to 20MB/s.


I know that the writes for a SLOG device are basically 4k syncronous writes (queue depth=1).
What i also know is that size doesn't matter here (maximum should be 50% of the RAM -> SSD size 16GB)

Some SSDs i found:

-Samsung 840 Pro or Evo
-OCZ Vector 150


Cheers

Thomy

 

[Ericloewe]

You'll definitely want something with backup capacitors. Something like the Intel S3700

 

[Thomymaster]

Hi

OK thanks für the reply but für what do i need backup capacitors? In the evnt if a power failure the SSD can weite the contents oft uts cache. But what abouz normal consumer SSDs or even HDDs, they don't habe a cache at all. Ok the contents if the in-flight data to the SLOG are list but not the entire content of the SLOG that zas not been commited to the pool

 

[Ericloewe]

Hi

OK thanks für the reply but für what do i need backup capacitors? In the evnt if a power failure the SSD can weite the contents oft uts cache. But what abouz normal consumer SSDs or even HDDs, they don't habe a cache at all. Ok the contents if the in-flight data to the SLOG are list but not the entire content of the SLOG that zas not been commited to the pool

If you're going to worry about having an ZIL at all, it's a bit weird to say "Oh, I can tolerate some data loss". The effects of not using SYNC or not providing a proper SLOG device are quite similar. Even a bit of corruption can ruin your day.

 

[Thomymaster]

OK i understood, so the backup capacitors will enable the SSD in case of a power failure to write the contents in its RAM to the NVRAM (the actual storage).

In means of speed and capacity, what to you recommend (am i right with the 4k syncronous qd=1 theory when speaking about SLOG)?

 

[Ericloewe]

OK i understood, so the backup capacitors will enable the SSD in case of a power failure to write the contents in its RAM to the NVRAM (the actual storage).

In means of speed and capacity, what to you recommend (am i right with the 4k syncronous qd=1 theory when speaking about SLOG)?

The Intel s3700 series is probably a good choice.

 

[HoneyBadger]

The Intel s3700 series is probably a good choice.

I'd say it's the best choice for a SATA SLOG you can buy new that isn't completely absurdly priced. It's still a lot more expensive than your general cheap MLC/TLC drives, but it's nowhere near the cost of enterprise SLC or battery-backed RAM.

 

[Thomymaster]

Hi

OK an S3700 costs 192€ for 100GB, maybe i found an alternative.

Crucial M550 (same hardware as the M500)
-Marvell 88SS9189 Controller (S-ATA 3.1)
-3 years warranty
-20NM MLC
90000/75000 4K (360/300) for a 128GB drive, bigger drives have mir IOPS
-DEVSLEEP support
-Power-Loss Protection (Backup-Capacitors)
-128GB 78€

What do you think about this one (especially due to the power-loss protection feature)?

 

[Ericloewe]

Hi

OK an S3700 costs 192€ for 100GB, maybe i found an alternative.

Crucial M550 (same hardware as the M500)
-Marvell 88SS9189 Controller (S-ATA 3.1)
-3 years warranty
-20NM MLC
90000/75000 4K (360/300) for a 128GB drive, bigger drives have mir IOPS
-DEVSLEEP support
-Power-Loss Protection (Backup-Capacitors)
-128GB 78€

What do you think about this one (especially due to the power-loss protection feature)?

Turns out it doesn't have true power loss protection - only for its internal metadata. This makes it a poor choice.

 

[kobazik]

The Intel s3700 series is probably a good choice.

Will two Intel S3700 100GB mirrored slogs be enough performance wise for a pool of mirrored 6x 1TB 5400rpm (WD10SPCX) and/or 6x 500GB 7200rpm drives? I'm looking for a good pair of mirrored ZIL SSDs to help with iSCSI workloads that will be used by an ESXi server. In addition my FreeNAS box will have 4x 1Gbit network ports so they cab be used for iSCSI MPIO.

 

[cyberjock]

iSCSI is async, so a ZIL is pointless unless you also meant to add to your post that you plan to set sync=enabled.

 

[kobazik]

I'm aware of that thanks :) My question is still open. Will two S3700 mirror be fine to provide enough speed for 6x 7200rpm pool? Or faster pool that will saturate ~500MB/s seq. writes?

 

[joeschmuck]

Just to jump in and stir up the pot a little... From my perspective, what is adding a different SSD going to gain you with respect to speed? Nothing as far as I can tell. Someone correct me if I'm wrong. That ADATA model you referenced is pretty damn fast.

 

[kobazik]

AFAIK sustained write speed is important for slog. Consumer grade SSD have great seq. read/writes but performance drops quiet badly after few minutes.

 

[jgreco]

I've been using a LSI2208 w/ BBU against some crappy 2.5" HDD's for SLOG and they sustain 50Mbytes/s (underlying drive speed) pretty much forever, except for the first few seconds where the RAID controller's write cache pretty much sucks it down as fast as it can go. Totally impractical costwise of course ;-)

 

[joeschmuck]

AFAIK sustained write speed is important for slog. Consumer grade SSD have great seq. read/writes but performance drops quiet badly after few minutes.

In my opinion, the write speed is more than sufficient for the bandwidth you are trying to achieve. You never mentioned your hardware setup, mind sharing that as well?

 

[jgreco]

It isn't really about write speed, though... it is latency that's the killer.

 

[joeschmuck]

It isn't really about write speed, though... it is latency that's the killer.

But latency can be very loosely related to IOPS which is what I was really referring to in my mind, not throughput.

Here is what I pulled up for the Adata SX900: Write Ave: .035ms, Write Max: 25.618ms, Read Ave: .186ms, Read Max 3.191ms (there are better values, it depends on the firmware in the SSD, I listed the worst case values) (From This site)

And the S3700: Write Ave: 1.28ms, Write Max: 26.21ms, Read values not listed. (From TomsHardware.com)

Both tests were IOmeter however different people, different testing system so take it for what it's worth, just an example. I mean, the Intel spec sheet is measured in microseconds, not sure what to make of that either. I'm not saying the ADATA SSD is better than the Intel SSD, but it seems like it on paper, with the exception of possibly the capacitor thingy, I've done enough speculating for today.

 

[cyberjock]

I think what jgreco is saying is that the benchmark values aren't what they seem with regards to slog usage. The Intel S3700s seems to be very good drives for slog. Other drives by other manufacturers that make claims to be superior in performance to the S3700 in every single way have been found to be absolutely terrible for slogs (in some cases so badly that the users were better off without an slog).

 

[Ericloewe]

The Intels are focused on write consistency. That may be the cause of their superiority.