On 2011-07-01, at 12:54 PM, Jianwei Liao liaot...@163.com wrote:
Though I have read the lustre manual roughly, but i am wondering about
the MDS operation flow:
1) Clients get the layout and capabilities from MDS, then do IO
operation. While the client modify the stripes, then the modification
time(Mtime) should be updated. Who(clients or OSSs) and When(after
closing the file or after the modification ) does send the update
request to MDS.
The mtime value is distributed over all of the servers that store part of the
file. When clients write data to an OST (as each RPC is sent) the mtime+ctime
from the client is stored on that OST object. If the client is setting
timestamps on the file, the mtime+ctime from the client is stored on the MDT
inode.
When the client is retrieving the mtime, it accesses the MDT inode and all OST
objects where the file is striped and uses the mtime on the node with the
newest ctime.
2) I have read several documents, but about unlinking a file, after
unlinking all stripes, who does send the message about all stripe have
been unlinked. One says, it is the clients who want to delete that
file (in the manual), but another one says, it is the OSTs(in Xyratex
Lustre Architecture Priorities Overview)?
The manual is correct. Don't be confused by the Architectural Priorities
document. That only contains features which do not exist yet.
3) During the creation, the MDS may ask the OSSs to allocation the
available stripes, then the clients can write or append the data. But
who does keep the information of available space in the stripe?
The client and OST continually negotiate how much space is available for it to
keep in the client writeback cache, so that the client does not cache unwritten
data that cannot fit on the OST. That is called space grant.
For example, while all stripe are used up, so, another new stripe(or
stripes) is needed, how do things go in this situation? (first who find
there is no available space, and then send request to MDS to allocation
a new stripe?)
There is currently no mechanism for adding more stripes to a file if one of the
OSTs is full. Each file can write to a stripe until the OST has no more space.
This is unlike other distributed filesystems where a stripe is a fixed or
maximum sized unit of space.
4) Is the opened_file list kept in the acitve MDS' memory?
Yes. There is one such list per client.
Cheers, Andreas
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