What is best use of spare SSD: read or write cache?

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What is best use of spare SSD: read or write cache?

Post by TomUnderhill » Wed Oct 10, 2012 8:43 am

By way of introduction, I own a short-run publishing company and have been producing limited-edition books such as memoirs, corporate histories, training manuals, fine art books and photography books. My projects range from several hundred MBs to hundreds of GBs. I occasionally produce videos, ranging in size from tens of minutes to several hours. I spend most of time in the Adobe CS6 suite. I also dabble in PHP and MySQL programming.

My current rig:

MacPro3,1
2x 3.0GHz quad-core Xeons
24GB RAM
Dual video cards - three panels (2x 20" and 1x 30")
1x 128GB SSD (boot)
4x 1TB HDD (internal, currently in a stripe managed by SoftRAID)
1x eSATA 2x 1TB HDD
Various network storage and several other Macs

I recently recovered from a near-death experience brought on by the boot SSD that just passed the 5500 hour mark and a case of "old firmware" induced freezes and indiscriminate storage errors. Flashing the drive fixed the problem but has brought me to re-examine my storage situation.

This is where ZFS comes in. I'm planning the reconfig while I'm moving data around and need some advice.

I have a spare 64GB SSD that I'm looking to use as either a write or read cache. Which will provide the best bang, given that most individual project files are smaller than 1GB.

Thanks for your guidance and experience. I'm looking forward to extending my tech experience!
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Re: What is best use of spare SSD: read or write cache?

Post by si-ghan-bi » Wed Oct 10, 2012 10:02 am

I think that the read cache (L2ARC) gets cleaned every time you reboot. Not only that, it's pretty slow to get filled.
If you don't leave your Mac on for one week at time, better use it for write cache, or split into two partitions and use them for both caches.
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Re: What is best use of spare SSD: read or write cache?

Post by TomUnderhill » Wed Oct 10, 2012 12:20 pm

Thanks much. I've got at least another four hours to contemplate this...

I might just swap the "new" 128GB SSD that's currently in my Mac mini server for this 64GB one I just swapped out for the cache. If I do, I'll give read and write each 64GB to play with.

If I could find a realistic way to speedily attach drives to the mini I'd drop both SSDs in there and use that machine as a backup/file/near-line storage server. The mini runs 24/7... my Mac Pro is an electricity-hungry demon.

With the mini it's just too painful to contemplate using USB2!
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Re: What is best use of spare SSD: read or write cache?

Post by grahamperrin » Wed Oct 10, 2012 1:00 pm

TomUnderhill wrote:… 64GB SSD that I'm looking to use as either a write or read cache. …


MLC or SLC?

If it helps: a recent answer to a question in Stack Exchange, For L2ARC and ZIL: is it better to have one large SSD for both, or two smaller SSDs?
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Re: What is best use of spare SSD: read or write cache?

Post by si-ghan-bi » Wed Oct 10, 2012 1:54 pm

You have a 4 TB array and you want more than 64 GB cache? Keep the big one in the iMac and leave the 64 for the cache, it's already a lot.
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Re: What is best use of spare SSD: read or write cache?

Post by TomUnderhill » Sun Nov 04, 2012 4:35 am

I've been running my 4TB array with the 64GB L2ARC read cache on my MacPro production workstation for a couple of weeks. Appears that si-ghan-bi was right about the cache cleansing on reboot. For a machine that doesn't run 24/7 for long periods of time, the L2ARC pathway isn't right for me.

Haven't had a chance to move it over to a write cache, but Apple's Fusion Drive changes the trail I might follow (or blaze).

I'd love to hear how a L2ARC enhances a machine that is predominantly on full-time.
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Re: What is best use of spare SSD: read or write cache?

Post by si-ghan-bi » Sun Nov 04, 2012 12:23 pm

TomUnderhill wrote:For a machine that doesn't run 24/7 for long periods of time, the L2ARC pathway isn't right for me.

Haven't had a chance to move it over to a write cache, but Apple's Fusion Drive changes the trail I might follow (or blaze).

I'd love to hear how a L2ARC enhances a machine that is predominantly on full-time.


On the other hand, using it as ZIL may not help if you don't have many synchronous writes. Actually, I think ZIL would help on very specific cases: unless you already know you need it, you probably don't need it.
Concerning the L2ARC, I have an external FW800 pool I use for movies, audio, and all other data except what I have in my user profile folder. I have never seen the USB2 flash L2ARC used except during operation of my virtual machines (and in that situation was used heavily). If you have only data in your pool, skip the L2ARC altogether and use that space for actual data.
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Re: What is best use of spare SSD: read or write cache?

Post by TomUnderhill » Sun Nov 04, 2012 10:56 pm

si-ghan-bi wrote:I have never seen the USB2 flash L2ARC used except during operation of my virtual machines (and in that situation was used heavily). If you have only data in your pool, skip the L2ARC altogether and use that space for actual data.


Rather than let the SSD sit there effectively being idle, I'm planning on yanking it and playing with a Fusion Drive setup.

Wonder if I can get the fancy drive icon or if that will require even more work?!
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Re: What is best use of spare SSD: read or write cache?

Post by audiophil » Wed Nov 07, 2012 10:45 am

Unless you do something on that pool that constantly hits the spindle disks hard, a ZIL is probably not going to speed things up much, if at all. Your probably better off using it for something else.

Everything I have read seems to indicate that using a MLC ssd in a ZIL may prematurely kill it. Unlike a L2ARC a lot of write IO will be going through it.

. . . .I would eventually like to see some benchmarks for use of a ZIL w/ a decent SSD (as in not anemic on write io); and how it effects usage on ZFS. It might be convenient for pumping up performance in cases where someone has a lot slower individual drives (etc its of USB and firewire devices). Anyone bothering to show off benchmarks for ZIL performance is using it with relatively high end hardware in demanding situations. .(database, VM storage, etc).
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Re: What is best use of spare SSD: read or write cache?

Post by TomUnderhill » Wed Nov 07, 2012 2:12 pm

I'm going to try something new with my disks. So...

Just broke apart my three drive RAIDZ with 64GB SSD as L2ARC. Didn't see any value with it there.

Just for fun I built a Fusion Drive out of the SSD and one of the 2TB HDDs out of my RAIDZ. Kinda cool.

Now I'm building a stripe of mirrors... or is it the other way around?!

My next new frontier... think I'm going to install the 64GB SSD into an iMac. Its current hard drive only has 48GB used.
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