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We are running 3 PHP web servers with the web directory (including the php files to be ran locally) pulled from a their own local SSD drive. We are deciding wheter to install a SAN to server our web directory from. The main reason for the switch to the SAN is to keep the files in sync and because the SAN is much more stable.

Does anyone have experience with the latency and throughput of a SAN vs a local SSD? Or what kind of performance loss/gain we will get.

Willemk
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  • People are going to need a lot more information to answer your question. A SAN can be much faster than a single local SSD but it really depends on the SAN and it's disk configurations, interconnect, etc. What sort of SAN you can get is going to depend on your budget. – 3dinfluence Nov 24 '10 at 20:55

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Unless your web-server is much busier or larger than is typical, the performance difference shouldn't be significant. The few areas where an SSD may outperform is if you're serving tens to hundreds of millions of files through the web and they're all on the same file-system. In that case the low latency of an SSD will shine through.

In the more typical "tens to hundreds of thousands" serving case the increased latency of the SAN should be markedly less than the latency already incurred by serving over the WAN. If you are concerned about latency, make sure your SAN contains lots of spindles instead of lots of space.

sysadmin1138
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  • Right on. Wonder if he's running a database in behind that is on the SAN? that might make a difference. – hookenz Nov 24 '10 at 23:43
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You're not clear but I think you've got the SSD in one server, sharing it to the others via some form of network share - is that right?

If that's the case then unless it's one hell of a SAN the SSD will be significantly faster in terms of both latency and throughput for the server that contains the SSD. The other machines getting at this data over the network will be limited by that network link and the sharing capabilities of the machine with the SSD - even with the fastest network setup this will be much slower than the SSD and is likely to be much slower than a SAN.

So if my assumption about your usage is right then you'll have one very quick server and two much slower servers (if they do perform a lot of disk work anyway, it's irrelevant if not).

If you do a lot of disk work and want consistent performance then moving to a SAN will help with that, no single server is likely to have quicker disk access than another - and it's likely to be perfectly adequate for most needs too.

Chopper3
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  • Update: Each server has their own SSD – Willemk Nov 24 '10 at 21:06
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    Thanks for clarifying. The SSD will be faster but what do you mean by "the SAN is much more stable", what stability issues have you had with SSDs? – Chopper3 Nov 24 '10 at 21:11
  • We've had an SSD (server has a single drive) crash in each server about once every 8 months. – Willemk Nov 24 '10 at 21:15
  • Try EFDs, not consumer SSDs. – Chris S Nov 24 '10 at 21:21
  • @Willemk - are you seeing a lot of disk activity with the SSDs? – Chopper3 Nov 24 '10 at 21:23
  • We are doing a small amount of disk activity: iostat gives 17TPS, 27Reads/Sec, 970Writes/Sec – Willemk Nov 24 '10 at 21:37
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    Oh dude, you won't miss the speed of those SSDs at all. – Chopper3 Nov 24 '10 at 22:04
  • Throughput of a local SSD is going to be bounded by the connection technology used, most likely SATA 3Gbps simplex in this case. A mid-spec fibre channel 4GFC is going to give you 3Gbps duplex. Unless I'm missing something, the mid-spec FC SAN edges out the SSD on throughput. – Chris Thorpe Nov 25 '10 at 08:00
  • @Chris, the SATA could be 3 or Gbps, the SAN 4 or 8Gbps but the question was about latency and throughput - and while I spend all my life tuning SANs to give the best performance I can (and I do frequently have arrays that will outpace SSDs) very often SANs won't get close to the latency of even a single SSD and frequently not even enough to flood 3Gbps of a 4Gbps pipe - so yes a SAN *could* be faster but in this particular case I'd imagine any SAN this dude buys will be slower overall than his SSDs. This is coming from a man that generally hates SSDs too by the way :) – Chopper3 Nov 25 '10 at 12:48
  • Just trying to get my head around it - We're finding situations regularly now where SSDs may be a viable option but I'm wary of trending from shared storage back to local storage without compelling reasons. How about SSDs served from a SAN? – Chris Thorpe Nov 25 '10 at 19:06
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SSD might help with log files on a very busy system. but is probably a bad use of resources.

Depending on the size and access distribution of your web site, you may find that most of the data is served from buffer cache in memory. In this case disk access statistics are relatively insignificant.

BillThor
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"SAN" can mean a lot of things. And if you're talking about Amazon AWS, there are provisioned IOPS to consider.

If you are provisioning a single server in a new server-room in a new building, locally attached is fine, and way less cost.

Your data synchronization needs are a separate concern, and you should ask that as a separate question IMO. Software can solve those issues. The sync requirement alone wouldn't justify a SAN.

For throughput, you should also consider using CloudFlare's entry-level free service to speed up access, if it's publically available on the internet. Which isn't clear either in your OP.

Kind Contributor
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