Understanding UBIFS flash overhead
dsaxena at plexity.net
Fri Oct 10 17:00:36 EDT 2008
I'm working on getting UBIFS going on the OLPC XO and am trying to
understand what I am seeing in regards to file system size.
I have partitioned the 1GiB mtd device into a 32MiB JFFS boot partition
for OFW, a 128KiB partition to hold the Redboot partition table,
and the remainder (991.625) for use by UBI.
The NAND device has 128KiB EBs with a 2KiB pages and we are running w/o
sub-page writes. Plugging this into the overhead calculation, we get:
SP PEB Size 128KiB
SL LEB Size 128KiB - 2 * 2KiB = 124 KiB
P Total PEBs 991.625MiB / 128KiB = 7933
B Reserved PEBsg 79(1%)
O Overhead= SP - SL 4KiB
UBI Overhead = (B + 4) * SP + O * (P - B - 4)
= (79 + 4) * 128Kib + 4 KiB * (7933 - 79 - 4)
= 42024 KiB
= 329.3125 PEBs (round to 329)
This leaves us with 7604 PEBs or 973312KiB available for user data.
At boot up, I see:
UBIFS: file system size: 995237888 bytes (971912 KiB, 949 MiB,
UBIFS: journal size: 9023488 bytes (8812 KiB, 8 MiB, 72 LEBs)
Filesystem Size Used Avail Use% Mounted on
mtd0 822M 242M 581M 30% /
I expect some overhead, but I'm really wondering where over 100MiB of
This is 2.6.25 with UBI 2.6.25 tree merged in. Does that tree have
any bugfixes from 2.6.26+ backported?
_____ __o Deepak Saxena - Living CarFree and CareFree (o>
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----- ( )/ ( ) for the future of the human race." -H.G Wells V_/_
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