[lustre-discuss] Read/Write on specific stripe of file via C api

[Andreas Dilger]

On Sep 18, 2024, at 10:47, Apostolis Stamatis <el18034 at mail.ntua.gr<mailto:el18034 at mail.ntua.gr>> wrote:
I am trying to read/write a specific stripe for files striped across multiple OSTs. I've been looking around the C api but with no success so far.


Let's say I have a big file which is striped across multiple OSTs. I have a cluster of compute nodes which perform some computation on the data of the file. Each node needs only a subset of that data.

I want each node to be able to read/write only the needed information, so that all reads/writes can happen in parallel. The desired data may or may not be aligned with the stripes (this is secondary).

It is my understanding that stripes are just parts of the file. Meaning that if I have an array of 100 rows and stripe A contains the first half, then it would contain the first 50 rows, is this correct?

This is not totally correct.  The location of the data depends on the size of the data and the stripe size.

For a 1-stripe file (the default unless otherwise specified) then all of the data would be in a single object, regardless of the size of the data.

For a 2-stripe file with stripe_size=1MiB, then the first MB of data [0-1MB) is on object 0, the second MB of data [1-2MB) is on object 1, and the third MB of data [2-3MB) is back on object 0, etc.

See https://wiki.lustre.org/Understanding_Lustre_Internals#Lustre_File_Layouts for example.

To sum up my questions are:

1) Can I read/write a specific stripe of a file via the C api to achieve better performance/locality?

There is no Lustre llapi_* interface that provides this functionality, but you can of course read the file with regular read() or preferably pread() or readv() calls with the right file offsets.

2) Is it correct that stripes include parts of the file, meaning the raw data? If not, can the raw data be extracted from any additional information stored in the stripe?

For example, if you have a 4-stripe file, then the application should read every 4th MB of the file to stay on the same OST object. Note that the *OST* index is not necessarily the same as the *stripe* number of the file.  To read the file from the local OST then it should check the local OST index and select that OST index from the file to determine the offset from the start of the file = stripe_size * stripe_number.

However, you could also do this more easily by having a bunch of 1-stripe files and doing the reads directly on the local OSTs.  You would run "lfs find DIR -i LOCAL_OST_IDX" to get a list of the files on each OST, and then process them directly.

3) If each compute node is run on top of a different OST where stripes of the file are stored, would it be better in terms of performance to have the node read the stripe of its OST? (because e.g. it avoids data transfer over the network)

This is not necessarily needed, if you have a good network, but it depends on the workload.  Local PCI storage access is about the same speed as remote PCI network access because they are limited by the PCI bus bandwidth.  You would notice a difference is if you have a large number of clients and they are completely IO-bound that overwhelm the storage.

Cheers, Andreas
--
Andreas Dilger
Lustre Principal Architect
Whamcloud







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