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I'm interested in getting a dedicated server with a SSD, probably Corsair Force series CSSD-F120GB2-BRKT.

It'll be an Linux server, most likely running XFS (but I'm open to other alternatives).

Many articles (as well as the Corsair website) all suggested that it's necessary to "4K align" the file system in order to get the highest performance from SSD.

What does "4K align" means? How do I do that?

On a related note, I'm using this server to run a website that's heavy on RDBMS usage. Is a SSD like Corsair Force truly persistent? If I flush data to the SSD is it truly written to the SSD or does the SSD cheat by caching the data?

Thanks.

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2 Answers 2

up vote 5 down vote accepted

yes you need to align - ssds [more to 32kB or 64KB than 4kB alignment - depending on underlying flash] and big hard-disks [usually 1.5TB or larger, to 4KB boundaries] and disks in stripping raids [RAID1, RAID5, to strip-length probably 32-128kB] should be aligned too.

why to align? read here.

how to align? check this.

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32K or 64K for SSD? Are you sure? The Corsair site (corsair.com/products/ssd_force) uses 4K as their benchmark for performance: "Random 4K write performance of 50,000 IOPS (4K aligned)" Can you explain why SSD needs 32KB or 64KB alignment instead of 4K? –  Continuation Oct 5 '10 at 7:10
5  
Because the discs are 64kb aligned. They use a 4kb benchmark because some databases do that, but the discs themselves use 64kb blocks. –  TomTom Oct 5 '10 at 8:28
    
@TomTom SSDs aren't discs, there are no spinning disc-shaped pieces of metal inside of them, the d stands for drive –  ptman Oct 5 '10 at 12:11
    
No, but SSD are still organized in blocks of 64kb / 32kb. A block can only be deleted COMPLETELY - so a change in one small part means loading the block and rewriting ig completely. Try reading some facts before making stupid comments. –  TomTom Oct 5 '10 at 16:44

They probably mean set the block size to 4k. http://en.wikipedia.org/wiki/XFS#Variable_block_sizes

I think the default used to be 512Bytes for a lot of file systems but people moved to using 4k. There should be a way of setting it when you create the filesystem.

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-s according to the mkfs.xfs(8) man page. –  Ignacio Vazquez-Abrams Oct 5 '10 at 6:37

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