Dear Nail,
in message <18346.39756.292908.58065@xxxxxxxxxxxxxx> you wrote:
>
> <quote>
> The second improvement is to remove a memory copy that is internal to the MD driver. The MD
> driver stages strip data ready to be written next to the I/O controller in a page size pre-
> allocated buffer. It is possible to bypass this memory copy for sequential writes thereby saving
> SDRAM access cycles.
> </quote>
>
> I sure hope you've checked that the filesystem never (ever) changes a
> buffer while it is being written out. Otherwise the data written to
> disk might be different from the data used in the parity calculation
> :-)
Sure. Note that usage szenarios of this implementation are not only
(actually not even primarily) focussed on using such a setup as
normal RAID server - instead processors like the 440SPe will likely
be used on RAID controller cards itself - and data may come from
iSCSI or over one of the PCIe busses, but not from a normal file
system.
> And what are the "Second memcpy" and "First memcpy" in the graph?
> I assume one is the memcpy mentioned above, but what is the other?
Avoiding the 1st memcpy means to skip the system block level caching,
i. e. try to use DIRECT_IO capability ("-dio" option to xdd tool
which was used for these benchmarks).
The 2nd memcpy is the optimization for large sequential writes you
quoted above.
Please keep in mind that these optimizations are probably not
directly useful for general purpose use of a normal file system on
top of the RAID array; they have other goals: provide benchmarks for
the special case of large synchrounous I/O operations (as used by
RAID controller manufacturers to show off their competitors), and to
provide a base for the firmware of such controllers.
Nevertheless, they clearly show where optimizations are possible,
assuming you understand exactly your usuage szenario.
In real life, your optimization may require completely different
strategies - for example, on our main file server we see such a
distribution of file sizes:
Out of a sample of 14.2e6 files,
65% are smaller than 4 kB
80% are smaller than 8 kB
90% are smaller than 16 kB
96% are smaller than 32 kB
98.4% are smaller than 64 kB
You don't want - for example - huge stripe sizes in such a system.
Best regards,
Wolfgang Denk
--
DENX Software Engineering GmbH, MD: Wolfgang Denk & Detlev Zundel
HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany
Phone: (+49)-8142-66989-10 Fax: (+49)-8142-66989-80 Email: wd@xxxxxxx
Egotist: A person of low taste, more interested in himself than in
me. - Ambrose Bierce
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