raid0 and iscsi
Hello Neil,
Thanks for the reply.
I was working on the internet scsi(iscsi drivers provided by intel) and software
raid configuration for my thesis in Storage Area Network.
The problems right now i am facing is
1> For a local disk write for 512 mb in DELL 1.2 Ghz it takes me 1 min. 2> For a single iscsi device write, the transfer of data takes place at 4000 bytes/sec.
2> For raid configuration, the rate is pretty slow ( 40 bytes/sec). One thing i have observe is that write to one of the device( from a total of 2 device) takes for few minutes and then stop.
I was trying to figure what routines in the operating system code, I might need
to look into to understand a problem if there is one.
Some more problems.
If one of the network comes down, the mkfs for the raid , hangs.
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A raid0 array can be made of a number of drives of differing sizes. To accomodate this we divide the address space into several blocks. The first block is striped across all drives to the size of the smallest. The next block is striped across the remaining drives to the size of the next smallest, etc.
Is there a relation between chunk size and block size of raid. Is this similar to buffer size( write(fd,&buffer,buffer_size) and block size of filesystem.
-Jay
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===== Original Message From Neil Brown <[email protected]> =====
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Hello,
I was testing the raid0 configuration for the iscsi device (
/dev/sdband/dev/sdc).For writing a 512MB in a LAN environment it was taking 24 hrs. i was going through the source code of
raid0.cand certain things doesn't made sense to
me.
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1> what role does the Hash bit play.
A raid0 array can be made of a number of drives of differing sizes. To accomodate this we divide the address space into several blocks. The first block is striped across all drives to the size of the smallest. The next block is striped across the remaining drives to the size of the next smallest, etc.
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Mapping from a virtual device address to block and thence a drive and offset is not straight forward. It requires a table search. The hash table helps accelerate this search.
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2>If my chunk size is 8 byte. Does that mean it will write 8 byte into
device 0
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and then into device 1. Is the write request to the disk in synchronous or asynchronous mode.
You cannot have an 8byte chunk size. 4K is the minimum. With an 8K chunk size and 2 devices,
sectors between 0 and 8K, 16k and 24K, 32K and 40K, 48K and 56K etc
are written to the first device.
sectors between 8K and 16K, 28K and 32K, 40K and 48K etc
are written to the second device.
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raid0 does not impose any synchronisation. Writes are only synchronous if the filesystem waits for them. raid0 never waits.
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3> Is
wite_disk_sbinmd.cresponsible for writing into disk??. Is
fsync_dev
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responsible for synhronous write ??. If so can i change to asynchronous
write.I
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will really appreciate if somebody can tell me what all routines I need to
go
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through to figure out the functionality of raid0 behaviour.
write_disk_sbis for writing the raid superblock to disk. It doesn't happen often.fsync_devis fairly irrelevant. You can safely ignore it.I hope that helps.
NeilBrown
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Thanks
Jayshankar-
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