Thoughts on RAID and SSDs

[Christopher]

Well now that I am mostly ready to build my new workstation I looked further into the matter and found a lot of disturbing stuff. I can only speak for windows and don't really know how much it is true for macs.

My thought was the following:

- Get 6 x 2 TB drives and put them in a RAID 10 (with an external raid controller) for image storage and live data.  Great works

For Scratch disk I thought of two ways:

- The cheap way get 4 HDDs in RAID 0. Great Works

- The expensive way use 2 or three 60-80GB SSDs in RAID 0. That sounds great doesn't it ? Well it sounds great but would have been completely stupid if I chose normal SSDs. So far the important TRIMM code does not work in a RAID array. So over time one would loose a lot of speed because the SSDs in the RAID array could not restore them self. So far this is true for ALL MLC based SSDs. The only option would be to use SLC based SSDs. Let me think, 32Gb cost around 300EURs, I would need at least 120GB which would be around 4 drives, which would be to spend 1200 on scratch. Compared to 150 on normal drives. Well just not gonna happen, will leave that test to someone else.


- Last the System drive. My original plan was to use three normal SSDs around 120GB in RAID 0 to get some nice speed and enough storage for temp panoramic files. Well will not happen, I would have the exact same problem as long as I use MLC SSDs. Just for fun I looked at what a RAID 0 with 3 x 120 GB SLC SSD would cost me. Well something around 800 per drive or 2400EURs in total. Well yes it certainly is fast, but I also know I probably can save that money and build a new computer in a year for 2400 which will be faster than it would be now. So not gonna happen

That leaves me with my current idea. I'm still open to all kind of infos and insights.

However I was thinking of getting a 120GB SSD (MLC) and get one 256GB SSD (MLC) Put all programs, system and files on the 120GB and put the Lightroom catalog and all current panoramic files onto the 256Gb. This would cost me around 800EURs which well, is the price for ONE 120GB SLC based SSD. Does that make sense ? Or am I missing something ?

Would it be for example to be smarter to use a 120GB SSD as system and instead of the 256GB SSD to use a 4 HDD RAID 0 array ?

Sorry that I have brought up so many questions, but I thought it might be of interest for all of us.


Oh by the way, how does a MAC handle TRIMM commands ?

[Jon Meddings]

However I was thinking of getting a 120GB SSD (MLC) and get one 256GB SSD (MLC) Put all programs, system and files on the 120GB and put the Lightroom catalog and all current panoramic files onto the 256Gb. This would cost me around 800EURs which well, is the price for ONE 120GB SLC based SSD. Does that make sense ? Or am I missing something ?

Would it be for example to be smarter to use a 120GB SSD as system and instead of the 256GB SSD to use a 4 HDD RAID 0 array ?

Sorry that I have brought up so many questions, but I thought it might be of interest for all of us.


Oh by the way, how does a MAC handle TRIMM commands ?

Chris, I am no expert and I'm interested to see what others, more expert than I, have to say but..... I have just completed a new workstation this week and am fairly happy with it. Life is a series of trade off's isn't it!

I used an i7 950 on an ASUS MB wtih 12 Gb RAM. No overclocking yet. For drives I hummed and hawed much as you are doing and then finally settled on the following

1. Long term slow but redundant storage - DAS and NAS in RAID 5
2. Local computer storage for lightroom catalogs and RAW files - 2, 2 TB drives non-RAID
3, Rapid boot and system startup - 160 Gb Intel SSD - non raid
4. Swap drive - single velociraptor 10K

While this is not instantaneous access or speed on startup I have been impressed wtih the speed of the system. No slowdowns in lightroom or PS and this includes blending of multiple stacks of images simultaneously. I have done one fairly large stitching project and again was impressed with the speed. I've not done any formal benchmarking though.

[John.Murray]

With increasingly larger and larger drive capacities, RAID 10 and even 5 is turning out to be a dumb idea.  Why?  The Unrecoverable Error Rate has remained the same - currently 1 for every 12tb.  I've personally run into this twice in the past year, RAID 10 Arrays experiencing UREs while re-building.  In one case, we took our lumps and restored from a file backup - the time it would have taken to completley rebuild the array actually exceeded the time lost be restoring the previous days backup!!!

Here's an interesting article discussing these issues:

http://blogs.zdnet.com/storage/?p=162

I honestly have no idea what URE rates are for SSD drives, but I would expect them to be far lower.....  Nice catch on the unavailability of the TRIM command....

[Christopher]

With increasingly larger and larger drive capacities, RAID 10 and even 5 is turning out to be a dumb idea.  Why?  The Unrecoverable Error Rate has remained the same - currently 1 for every 12tb.  I've personally run into this twice in the past year, RAID 10 Arrays experiencing UREs while re-building.  In one case, we took our lumps and restored from a file backup - the time it would have taken to completley rebuild the array actually exceeded the time lost be restoring the previous days backup!!!

Here's an interesting article discussing these issues:

http://blogs.zdnet.com/storage/?p=162

I honestly have no idea what URE rates are for SSD drives, but I would expect them to be far lower.....  Nice catch on the unavailability of the TRIM command....

Well you are right, that one could argue that running two RAID 0 is more effective and fatser than one RAID 10, especially when it comes to rebuilding, however it only works as long as one drive of each array is failing.

When it comes to RAID and SSD I think we really are just at the beginning and perhaps we will be able to use TRIMM in the future when SSDs are used in a RAID, but I would never spend 1000EURs on drives which get slower and slower quit fast with a unkonwn factor if they ever would manage to know the TRIMM command.

[BernardLanguillier]

- The expensive way use 2 or three 60-80GB SSDs in RAID 0. That sounds great doesn't it ? Well it sounds great but would have been completely stupid if I chose normal SSDs. So far the important TRIMM code does not work in a RAID array. So over time one would loose a lot of speed because the SSDs in the RAID array could not restore them self. So far this is true for ALL MLC based SSDs. The only option would be to use SLC based SSDs. Let me think, 32Gb cost around 300EURs, I would need at least 120GB which would be around 4 drives, which would be to spend 1200 on scratch. Compared to 150 on normal drives. Well just not gonna happen, will leave that test to someone else.

Do you have clear stats about the typical size of your cached data?

I would go for a cache size that can handle 80% of the cases, and use a fast HD to cover the remaining 20% as a second cache.

Cheers,
Bernard

[Christopher]

Do you have clear stats about the typical size of your cached data?

I would go for a cache size that can handle 80% of the cases, and use a fast HD to cover the remaining 20% as a second cache.

Cheers,
Bernard

Well that is a good question, I think something around 60GB would be good for most cases, however even two 32 GB or 4 16Gb SLC SSDs will be around 600EUR.

[Plekto]

The way around it is to do what server farms do and stick to 250GB or so drives and have a LOT of them in a rack.  But that's space and power prohibitive.  It's much better, then, to use SSDs (or will be once their capacity to price ratio improves).

That said, my boot drive is a pair of raid 1 160GB drives.  It's small but a rebuild only takes me 30 minutes at most.  Other files are better off on single drives or backed up someplace.