Changing location of disk/filesystem cache

[Greg Freemyer]

Mandeep Sandhu <mandeepsandhu.chd at gmail.com> wrote:
>On Thu, Oct 10, 2013 at 5:00 PM, Ulka Vaze <ulka.vaze at l2it.com> wrote:
>
>> Hi,
>> disk cache is same as filesystem  cache. Also called buffer cache.
>> This is implemneted below fs layer.
>> It is basically a cache of disk blocks  mainatined in RAM. (In pages)
>> called buffers.
>>
>
>Ok. So this won't contain "files" but rather "blocks" many of which
>will
>represent a single file?

The buffer cache knows nothing about files.  It may happen to contain the blocks that correspond to a complete file, but the buffer cache doesn't have any way to know that.

>> The purpose of this cache is to improve performance as disk devices
>are
>> slow.
>> You can access this cache from the kernel.
>> Block layer accesses this from the request structure and commits
>blocks on
>> disk.
>> There are more layers in between like IOschedulers / SCSI etc.
>>
>
>Where does the mapping for file to disk pages/blocks exist? Is it in
>the
>inode or dentry entries or something else?

That is a very filesystem dependent question.  

In general dentry entries point to inodes and inodes point to blocks of pointers.  Those pointers point to actual data blocks.

You need to discuss a specific filesystem type to even start to discuss anything specific.

Ie. Does the fat filesystem have an equivalent of inodes?

>> How  does your device accesses files ?
>>
>
>The device itself runs stripped down version of a fairly recent Linux
>version (3.x). It has DMA capabilities to transfer content to/from the
>hosts memory from/to it's own.

If 3.x is really current you have 2 more options at least: dm-cache and bcache.

I don't know much about either, but they are going to be better options for I think than the traditional buffer cache.
Greg
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