Tracking the system calls for OSD write

[xinxin.shu@xxxxxxxxx (Shu, Xinxin)]

The system call is invoked in FileStore::_do_transaction().

Cheers,
xinxin

From: ceph-users [mailto:ceph-users-bounces@xxxxxxxxxxxxxx] On Behalf Of Sudarsan, Rajesh
Sent: Thursday, August 14, 2014 3:01 PM
To: ceph-devel at vger.kernel.org; ceph-users at lists.ceph.com
Subject: Tracking the system calls for OSD write

Hi,

I am trying to track the actual system open and write call on a OSD when a new file is created and written. So far, my tracking is as follows:

Using the debug log messages, I located the first write call in do_osd_ops function (case CEPH_OSD_OP_WRITE) in os/ReplicatedPG.cc (line 3727)

t->write(soid, op.extent.offset, op.extent.length, osd_op.indata);

where t is a transaction object.

This function is defined in os/ObjectStore.h (line 652)

void write(coll_t cid, const ghobject_t& oid, uint64_t off, uint64_t len,
               const bufferlist& data) {
      __u32 op = OP_WRITE;
      ::encode(op, tbl);
      ::encode(cid, tbl);
      ::encode(oid, tbl);
      ::encode(off, tbl);
      ::encode(len, tbl);
      assert(len == data.length());
      if (data.length() > largest_data_len) {
        largest_data_len = data.length();
        largest_data_off = off;
        largest_data_off_in_tbl = tbl.length() + sizeof(__u32);  // we are about to
      }
      ::encode(data, tbl);
      ops++;
    }

The encode functions are defined as a template in include/encoding.h (line 61) which eventually calls bufferlist.append in src/common/buffers.cc (line 1272) to insert the buffers from one list to another.

  void buffer::list::append(const list& bl)
  {
    _len += bl._len;
    for (std::list<ptr>::const_iterator p = bl._buffers.begin();
         p != bl._buffers.end();
         ++p)
      _buffers.push_back(*p);
  }

Since the buffers are "push_back" I figured that there must be a call to pop_front in ReplicatedPG.cc . But there is not pop_front associated with any write. This is where I am stuck.

At this point I have two questions:

1.       When does the actual file open happen?

2.       Where is the system call to physically write the file to the disk?

Any help is appreciated.

Rajesh
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