Re: SSD and non-SSD Suitability

[David Arendt <admin-/LHdS3kC8BfYtjvyW6yDsg@public.gmane.org>]

Hi,

On 05/29/10 10:45, Gordan Bobic wrote:
> David Arendt wrote:
>
>>>>>> 4) As the data gets expired, and snapshots get deleted, this will
>>>>>> inevitably lead to fragmentation, which will de-linearize writes
>>>>>> as they have to go into whatever holes are available in the data.
>>>>>> How does this affect nilfs write performance?
>>>>>>         
>>>>> For now, my understanding, nilfs garbage collector moves the live
>>>>> (in use)
>>>>> blocks to the end of logs, so holes are not created (it is correct?).
>>>>> However, it leads another issue that garbage collector process,
>>>>> which is
>>>>> nilfs_cleanerd, will consume the I/O.  This is major I/O performance
>>>>> bottle neck current implementation.
>>>>>       
>>>> Since this moves files, it sounds like this could be a major issue
>>>> for flash media since it unnecessarily creates additional writes.
>>>> Can this be suppressed?
>>>>     
>>> You can simply kill the nilfs_clearnerd after you mount the nilfs
>>> partition.
>>>   
>> If you use the latest nilfs_utils, killing nilfs_cleanerd is no longer
>> necessary. You can use mount -o nogc. This will not start
>> nilfs_cleanerd. Another possibility is to let nilfs_cleanerd start and
>> tweak min_free_segments and max_free_segments so that cleanerd will only
>> do cleaning if necessary.
>
> What about making the gc run only if the disk has been idle for, say,
> 20ms, unless min_free_segments is reached?
>

Well, this method would have the advantage that everything older than
protection_period would be freed up without having to much impact on the
system, so you would see the amount of free space. However the
disadvantage would be that cleaning would be done when no free space is
needed which could lead to shorter lifetime of flash devices.

> Gordan
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Bye,
David Arendt
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