linux-ext4 - Re: [RFC] fadvise: add more flags to provide a hint for block allocation

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Date:	Tue, 06 Mar 2012 09:53:29 -0800
From:	Sunil Mushran <sunil.mushran@...cle.com>
To:	Lukas Czerner <lczerner@...hat.com>
CC:	Zheng Liu <gnehzuil.liu@...il.com>, linux-fsdevel@...r.kernel.org,
	linux-ext4@...r.kernel.org
Subject: Re: [RFC] fadvise: add more flags to provide a hint for block allocation

On 03/06/2012 06:29 AM, Lukas Czerner wrote:
> However the file system do not have the information which part of the
> device it resides on is faster. It might be the beginning of the file
> system, but it might not be the case at all.


Think HSM and flash storage as the hot region. Remember these are
hints and not guaranteed to work in all cases.


> Moreover the flag which is stating that the file does not have to be
> allocated sequentially is not particularly helpful, I can not imagine
> people using it. Why would someone want to lower their performance ?
> Well, they might think that it will increase performance of the other
> files, but that is highly disputable and there are better solutions like
> using faster storage for the files that actually needs it.
>
> Additionally *_HOT* flag does not say anything about the allocation
> policy. It might be accessed often ,but no in sequential manner, or it
> can be written to a lot, it can be appended a lot, or it the content
> might be changed without changing its size etc... *Hot* might mean so
> many thing that this is just not useful for the file system. It would
> certainly be better to come up with something less esoteric which would
> actually address concrete user issues and help file system to deal with
> them better, like, I do not know, do not fsync/force allocation on
> rename maybe...(or whatever we are doing right now).

_HOT/_COLD is descriptive for allocation policy though fadvise() is
the wrong call as it pertains to access patterns.

Sunil
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