-------- Original Message --------
Subject: Re: What about storing more crc32 in the unused csum size for
metadata?
From: Duncan <1i5t5.dun...@cox.net>
To: <linux-btrfs@vger.kernel.org>
Date: 2015年01月12日 12:52
Qu Wenruo posted on Mon, 12 Jan 2015 10:32:30 +0800 as excerpted:
[New layout]
Take leafsize as 4K for example.
Sectors:
|---0---|---1---|---2---|---3---|---4---|---5---|---6---|---7---|
Sector 0: Csum of the leaf/node (32~4K)(not changed)
Sector 1: Csum of the first eighths of the leaf/node (32~512)
Sector 2: Csum of the second eighths of the leaf/node (512~1024)
....
Sector 7: Csum of the last eighths of the leaf/node (3584 ~ 4096)
OK, I've been up too long and should be sleeping, and it may be that's
affecting my logic, but AFAICT either you're missing something obvious,
or I am.
There are eight eights, and eight "sectors", but the first one is already
used, thus leaving seven available. You have the first eighth (ending at
byte 512*1) in sector one (zero-based so the second sector), the second
(ending at byte 512*2=1024) in sector two... the last (eighth) eighth
(ending at byte 512*8=4096) in sector seven.
Of eighths 3-7 in that ..., which eighth do you skip csumming, or is one
of them a full quarter instead of an eighth?
Oh, my fault, sector 1 should contains csum for the second eighth of the
leaf/node...
Since the first eighth csum can be calculated vice verse.
Thanks,
Qu
Of course if we could make them sevenths... But making it four quarters
instead of 7/8 is perhaps more reasonable, with the remaining three
sectors remaining unused, and if only one fails csum we still save 75% of
the leaf/node, instead of none of it, currently.
But maybe it makes complete sense as-is and I should be sleeping instead
of trying to make sense of what is clearly not making sense to me at this
point...
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