On May 26, 2010, at 12:18 PM, Gordan Bobic wrote:
I've got a somewhat broad question on the suitability of nilfs for
various workloads and different backing storage devices. From what
I understand from the documentation available, the idea is to
always write sequentially, and thus avoid slow random writes on old/
naive SSDs. Hence I have a few questions.
1) Modern SSDs (e.g. Intel) do this logical/physical mapping
internally, so that the writes happen sequentially anyway.
Could you explain that, as far as i know modern SSD's have 8
independant channels to do read and writes, which is why they are
having that big read and write speed and can in theory therefore
support 8 threads doing reads and writes. Each channel say using
blocks of 4KB, so it's 64KB in total.
Does nilfs demonstrably provide additional benefits on such modern
SSDs with sensible firmware?
2) Mechanical disks suffer from slow random writes (or any random
operation for that matter), too. Do the benefits of nilfs show in
random write performance on mechanical disks?
3) How does this affect real-world read performance if nilfs is
used on a mechanical disk? How much additional file fragmentation
in absolute terms does nilfs cause?
Basically the main difference between SSD's and traditional disks is
that SSD's have a faster latency, have more than 1 channel and write
small blocks of 4KB, whereas 64KB read/writes are already real small
for a traditional disk.
So a file system should benefit from the special properties of a SSD
to be suited for this modern hardware.
4) As the data gets expired, and snapshots get deleted, this will
inevitably lead to fragmentation, which will de-linearize writes as
they have to go into whatever holes are available in the data. How
does this affect nilfs write performance?
5) How does the specific writing amount measure against other file
systems (I'm specifically interested in comparisons vs. ext2). What
I mean by specific writing amount is for writing, say, 100,000
random sized files, how many write operations and MBs (or sectors)
of writes are required for the exact same operation being performed
on nilfs and ext2 (e.g. as measured by vmstat -d).
Isn't ext2 a bit old?
Of course i understand you skip ext4 as that obviously still has to
get bugfixed.
Many thanks.
Gordan
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