Hi Christina,
I answered some of your specific questions in line below:
Christina Patrick wrote:
Hi All,
I am doing a project where I need to implement simple prefetching in
pvfs. While I was going through the pvfs code, I couldn't understand
the following and hence have to ask the below questions:
1. What is the file descriptor assigned by pvfs? Does this file
descriptor correspond to actual descriptor (of the underlying
filesystem) ... for eg. that which is seen by /proc filesystem?
The file descriptor used on the client side has no relation to the file
descriptors used by the servers. The client's file descriptor is
assigned by the Linux kernel when a file is opened.
2. When I traced the I/O calls in pvfs v2.8.0, I found the following
function chain on the pvfs2 server side:
main
-> PINT_state_machine_continue
-> PINT_state_machine_next
-> PINT_state_machine_invoke
-> io_start_flow
-> job_flow
-> PINT_flow_post
-> fp_multiqueue_post
-> bmi_send_callback_fn
-> trove_bstream_read_list
-> alt_aio_bstream_read_list
-> dbpf_bstream_rw_list
-> issue_or_delay_io_operation
-> alt_lio_listio
-> pthread_create => alt_lio_thread
alt_lio_thread (new thread)
-> pread
Since, pvfs is a parallel file system, are all the file descriptors
assigned on all the servers the same? I saw one instance where the
servers used descriptors 12, 12 & 17 and another instance where all
the file servers used descriptor 12?
I think the previous question sort of answered that, but the servers get
their own file descriptors as needed; they have no relation to the
client's file descriptor. If the servers happen to match it is just
coincidence.
3. The most important question is how come all the file servers (the
function alt_lio_thread) are passed the same offsets and also the
total size of the data to be read? Shouldn't each file server be
passed a list of offsets corresponding to what is stored on its own
local disk (bcoz of the striping)? Where does this translation
actually take place and how does the server read in the appropriate
data in other words how is the striped data actually read?
The offsets used by the client have to get mapped to what makes sense
relative to each server's part of the data. The default striping unit
is 64 KB. This means that if you read at offset 64 KB, for example,
then it will actually correspond to offset 0 of the 2nd server's data.
It is pretty common to hit the same local offset on every server.
4. In order to implement prefetching on the server side, how do I have
to go about it? Can I do some shortcut method or do I need to
introduce some new state machine etc? I could really do with some help
here.
I really appreciate your help,
Thanks and Regards,
Christina.
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