From mboxrd@z Thu Jan  1 00:00:00 1970
From: Edward Shishkin <edward.shishkin@gmail.com>
Subject: Re: [PATCH 0/6] reiser4: discard support: "precise discard" aka padding
 of extents to erase unit boundaries.
Date: Mon, 15 Dec 2014 20:30:29 +0100
Message-ID: <548F36D5.4000402@gmail.com>
References: <1418418632-18396-1-git-send-email-intelfx100@gmail.com> <548CBFCA.60105@gmail.com> <1847370.hWhlfNze1m@intelfx-laptop>
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To: Ivan Shapovalov <intelfx100@gmail.com>
Cc: reiserfs-devel@vger.kernel.org

On 12/14/2014 09:03 AM, Ivan Shapovalov wrote:
> On Saturday 13 December 2014 at 23:38:02, Edward Shishkin wrote:	
>> On 12/12/2014 10:10 PM, Ivan Shapovalov wrote:
>>> So, this patchset uses your algorithm of padding/gluing extents to be discarded.
>>> I've added exact allocation primitives (is it what you've been talking about?
>>
>> So, we found out that the old bitmap primitives don't allow us to implement
>> precise discard, and I asked for new ones. Also we wanted to implement the
>> general case, when precise discard works for any pair (offset, unit).
>> Currently
>> it works only for ones determined by the function convert_lattice_params().
>> For some reasons (that I don't remember) I was sure, that the general case
>> should be obtained from this partial case by a small transformation of the
>> function discard_sorted_merged_extents().
>>
>> Well, I'll try to dive into details and generalize things on the holidays.
>> If generalization is impossible, then we'll switch to non-precise
>> discard every
>> time when convert_lattice_params() fails.
> I don't quite understand you here... Do you mean that some non-insane combinations
> of discard unit, discard offset and blocksize are not handled (but worth
> handling)?


Yes. As you can see, if (discard_offset % block_size != 0), then we
don't issue discard requests. Neither precise, nor non-precise.
It is hard to implement the precise discard support in the generic case 
straight away.


>
> TBH, convert_lattice_params() looks magic-ish to me...


OK, I'll try to provide some background (geometric minimum).

Edward.