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Is anybody here running SQL Server on solid-state drives? Have you found any specific optimization tips? I'm specifically interested in ways to reduce the frequency with which SQL Server performs small random write operations since they're the nemesis of SSD performance, particularly MLC SSD drives.

There are some obvious optimizations one can do, of course: read-heavy data should be served from the SSD, and write-heavy stuff should be left to traditional spinning disks. That includes transaction logs, naturally!

Given enough budget, of course, one would want to use SLC SSD disks like the X25-E or the Vertex Ex series or various enterprise-level offerings. But I'm also interested in tips that might benefit MLC SSD setups. I think that's an interesting area. One of my clients' clients has a small budget and a dataset that's grown immensely and they're facing a complete rewrite of close to a hundred queries in order to maintain a decent level of performance. However, I have a sneaking suspicion that less than $500 of RAM and SSD space might net them a bigger performance gain than thousands (possibly tens of thousands) of dollars worth of developer time.

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up vote 3 down vote accepted

Not sure what you mean by reducing the amount of small, random writes that SQL Server does. SQL Server writes out data pages only during checkpoints - so the only way to limit the number of writes is to change the checkpoint interval or don't so many IUD operations. Did you mean something else?

In all the implementations of SSDs that I've seen (a handful), it's kind of the opposite of what you're suggesting - the best use of SSDs seems to be for write-heavy transaction logs and tempdb - basically where's the biggest I/O subsystem bottleneck and stick the SSDs in there - as seek time and latency are reduced to a low constant.

Checkout this research paper that MS produced (unfortunately not hugely detailed on SQL Server specifics): Migrating Server Storage to SSDs: Analysis of Tradeoffs.

Hope this helps!

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Thank you for that link to MS' article. It is frustratingly short on specifics, isn't it? :) Unfortunately small random writes are indeed something that can give SSDs fits. In a nutshell, for even a small (ie, 4KB) write, the SSD must read an entire block into memory, modify it, and write it back out. It's just how current-gen flash memory works. Great SSD overview article: – John Booty Jul 1 '09 at 17:21

You can't modify SQL Servers IO charactaristics. Its basic unit of disk access, for data files, is an 8Kb page. It will write them mostly during a checkpoint, but will also lazy write them when it can.
SQL doesn't wait for writes to the data disk to complete before returning, it is only the log writes that must be completed. If you can keep only one database log on a disk then it will be sequential writes and will be fine on normal fast hard disks.
The performance hit from SQL's point of view is when it has to read the disks. If you can give it more memory then SQL will hold more data pages in memory, which is faster than any sort of disk, SSD or otherwise. Obviously you can also reduce the number of disk reads by creating appropriate indexes. I expect an SSD would also help with these reads because they are likely to be random and held up waiting for drive heads to move.
I don't know what database size we are talking about here, but have you might want to take a look at HyperOS. They make sata disks that are actually just a load of DDR2 ram sticks, with an SSD or 2.5 inch disk as a backup. The access pattern of the server won't matter a jot then. I wouldn't put the logs on anything like this though. Logs are what keep your data consistant, they need to go on a reliable medium and despite its back up SSD and battery and the server probably has a UPS etc, I would still feel un-easy about not having my logs on a real hard disk in some sort of failure tolerant RAID array.

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Small random operations are the nemesis of traditional disks, due to head seek latency... SSDs are great at addressing exactly this.

With long, sequential operations, standard disks perform quite good, so there would be no purpose in using SSDs (from a performance standpoint, of course).

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SSDs are fantastic at random read operations due to near-zero seek latency. They're less adroit at random write operations due to the fact that an SSD write operation involves reading an entire flash block (typically, 128KB), modifying the contents, and writing the entire block back out to flash. As for long, sequential operations, the better consumer-level SSDs (Intel, OCZ Vertex, Samsung) achieve well over 200MB/sec reads and 80MB-150MB writes, well above what a single spinning disk can produce. – John Booty Jun 26 '09 at 18:21
Are you sure? I don't understand why a write operation should involve reading a data block before writing it out again... data to be written should be in the computer's memory, shouldn't them? – Massimo Jun 27 '09 at 16:22
@Massimo: because the OS writes only a few bytes, but the SSD works in units (pages) of 128KB (usually). It can only write a page of 128KB, nothing less, nothing more. So when you modify let's say the middle of a page, the drive reads the whole page, updates the middle and then writes the new page usually somewhere else, while invalidating the old location. – Cristian Ciupitu Jun 30 '09 at 15:46
Cristian Ciupitu is correct. In some SSDs this is mitigated by an onboard cache (all the drives using the Indilinx controller have 64MB of cache, I believe) and perhaps also by the operating system's write caching, if enabled. Even a 64MB cache has its limitations, though - for a database server doing lots of writes, 64MB might not be enough. Firmware manufacturers don't release a lot of specifics, but one would assume the better firmwares (Intel, Indilinx) do some intelligent reordering/batching to keep small random writes within a 128KB page to minimize this overhead. – John Booty Jul 1 '09 at 17:18
From my understanding of the cache, it will save you a lot of these small writes you are so worried about. It doesn't even matter that much, since databases are designed to do a lot of linear read/writes. I bet SSD would still work better since its a linear read, not a seq one. Meaning there will still be gaps between data and SSD would remove the seek time. – Pyrolistical Jul 3 '09 at 17:19

Not enoug point here yet to add into the comment thread, but if you set the DB's page size / multi read count for anything on the SSD's to a multiple of the SSD's page size this should not be an issue.

I have not worked on SQL Server in a long time so I am not sure if these options are available there. I've been doing Oracle and DB2 for the last few years and this would solve your concerns as the DB would be properly tuned to the disk characteristics.

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I would recommend aligning partition where database files are stored.

I would also recommend deciding what goes on RAID 0 for perf (ldf and TempDB), and place critical data on RAID 1 (mdf).

Thirdly, you really should update drive's firmware as well as SATA controller firmware / drivers. By doing so you give the hardware company and their developers a chance to optimize perf for you.

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RAID 0 shouldn't ever be used for a database server. If a single drive fails then the database is down until the disk is replaced and the missing data restored from tape (this includes the log). – mrdenny Jul 7 '09 at 1:24
In a world where money is no object, everything should run on battery-backed L1 cache. In the banking industry, LDF file is just as important as the mdf. For scientific computing MDF is the only file that really needs to be 100%. – GregC Jul 7 '09 at 8:03

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