Re: btrfs performance - ssd array

[Austin S Hemmelgarn <ahferroin7@gmail.com>]

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On 2015-01-12 10:11, Patrik Lundquist wrote:
> On 12 January 2015 at 15:54, Austin S Hemmelgarn <ahferroin7@gmail.com> wrote:
>>
>> Another thing to consider is that the kernel's default I/O scheduler and the default parameters for that I/O scheduler are almost always suboptimal for SSD's, and this tends to show far more with BTRFS than anything else.  Personally I've found that using the CFQ I/O scheduler with the following parameters works best for a majority of SSD's:
>> 1. slice_idle=0
>> 2. back_seek_penalty=1
>> 3. back_seek_max set equal to the size in sectors of the device
>> 4. nr_requests and quantum set to the hardware command queue depth
>>
>> You can easily set these persistently for a given device with a udev rule like this:
>>    KERNEL=='sda', SUBSYSTEM=='block', ACTION=='add', ATTR{queue/scheduler}='cfq', ATTR{queue/iosched/back_seek_penalty}='1', ATTR{queue/iosched/back_seek_max}='<device_size>', ATTR{queue/iosched/quantum}='128', ATTR{queue/iosched/slice_idle}='0', ATTR{queue/nr_requests}='128'
>>
>> Make sure to replace '128' in the rule with whatever the command queue depth is for the device in question (It's usually 128 or 256, occasionally more), and <device_size> with the size of the device in kibibytes.
>>
>
> So is it "size in sectors of the device" or "size of the device in
> kibibytes" for back_seek_max? :-)
>
size in kibibytes, sorry about the confusion, I forgot to correct every 
instance of saying it was size in sectors after I reread the documentation.


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