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[RFD] xfs_fsr: Doubts related to xfs_fsr code

To: xfs@xxxxxxxxxxx
Subject: [RFD] xfs_fsr: Doubts related to xfs_fsr code
From: Somdeep Dey <somdeepdey10@xxxxxxxxx>
Date: Sat, 13 Sep 2014 22:37:53 +0530
Delivered-to: xfs@xxxxxxxxxxx
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Hi,

While studying the code and attempting to understand it, we have come up against certain doubts that have us in a slight fix. We've included the concerned
Âsections of the code below, along with our specific problem in each of theseÂ
Âsections. A little pointer in the right direction would be of great help.

(Source file : fsr_xfs_fsr.c)

****** Doubt number 1 ******
(line 331 onwards)
int main()Â

if (optind < argc) // If the command line input contains the XFSÂ
//filesystem name / file on which xfs_fsr needs to be run on
{
for (; optind < argc; optind++)Â
{
argname = argv[optind]; // save target which can be file or filesystem
if (lstat64(argname, &sb) < 0)
{ /* This system call returns a stat64 structure, and thus sets     Â
        * all fields in it.
* On success, zero is returned. ÂOn error, -1 is returned,Â
        * and Âerrno Âis set appropriately.
*/
fprintf(stderr,
_("%s: could not stat: %s: %s\n"),
progname, argname, strerror(errno));
continue;
}
// POSIX macros are defined to check the file type using the st_mode field
if (S_ISLNK(sb.st_mode)) // Check if path(argname) is aÂ
//symbolic link, if so link will be stat-ed and not file
{// Hence Âwe run stat64() and save the obtained stat structure
struct stat64 sb2;
if (stat64(argname, &sb2) == 0 && (S_ISBLK(sb2.st_mode) || ÂÂ
  S_ISCHR(sb2.st_mode)))
sb = sb2; // check if stat is a success andÂ
//if argname(path) is block device ? OR is character device?
}
________________________________________________________________________________

We understand that lstat64() and stat64() are used to see if target
Â(file/filesystem) can be stated and if yes then the structure is saved.Â
But we couldnât exactly understand its use and why both functions are used separately.
Usually the error could not stat: filename : is followed by Permission denied.
ÂIs this related to the root permissions i.e. accessibility ?

****** Doubt number 2 ******

(line 184 onwards)
static char *
find_mountpoint(char *mtab, char *argname, struct stat64 *sb)

while ((t = getmntent(mtabp))) {
if (S_ISDIR(sb->st_mode)) { /* mount point */
if (stat64(t->mnt_dir, &ms) < 0)
continue;
if (sb->st_ino != ms.st_ino)
continue;
if (sb->st_dev != ms.st_dev)
continue;
if (strcmp(t->mnt_type, MNTTYPE_XFS) != 0)
continue;
} else { /* device */
struct stat64 sb2;

if (stat64(t->mnt_fsname, &ms) < 0)
continue;
if (sb->st_rdev != ms.st_rdev)
continue;
if (strcmp(t->mnt_type, MNTTYPE_XFS) != 0)
continue;

/*
* Make sure the mount point given by mtab is accessible
* before using it.
*/
if (stat64(t->mnt_dir, &sb2) < 0)
continue;
}
________________________________________________________________________________
We just wanted to confirm if the basic working of the function is
1) To obtain a mount table pointer to the mount table (/etc/mtab/ or /proc/mounts).
2) ÂFor each entry in the mount table check if it is a directory or device and after
Âperforming various comparisons (checks) â (could you please elaborate on the checks
 performed), this function returns a pointer to the entry.Â

****** Doubt number 3 ******

(line 677 onwards)
static int
fsrfs(char *mntdir, xfs_ino_t startino, int targetrange)

For the followingÂ

__s32 buflenout;

fshandlep = jdm_getfshandle( mntdir );
if ( ! fshandlep ) {
fsrprintf(_("unable to get handle: %s: %s\n"),
    Âmntdir, strerror( errno ));
return -1;
}
________________________________________________________________________________

We are a bit confused about what is exactly the file handle being returned
Âby jdm_getfshandle().
Also what exactly is buflenout. Is it a structure field ?
________________________________________________________________________________

while ((ret = xfs_bulkstat(fsfd,&lastino, GRABSZ, &buf[0], &buflenout) == 0))
{
xfs_bstat_t *p;
xfs_bstat_t *endp;

if (buflenout == 0)
goto out0;

/* Each loop through, defrag targetrange percent of the files */
count = (buflenout * targetrange) / 100;

qsort((char *)buf, buflenout, sizeof(struct xfs_bstat), cmp);
________________________________________________________________________________

In the above code snippet we understand that the while loop will run for N(10)Â
passes and defragment top 10% of the defragmented files.
However we would appreciate if you could further explain the functions:

(ret = xfs_bulkstat(fsfd,&lastino, GRABSZ, &buf[0], &buflenout) == 0)

qsort((char *)buf, buflenout, sizeof(struct xfs_bstat), cmp);

We know that the sort function will be used to sort extents based on size and thenÂ
offset, but a bit more information on how it is exactly working will be reallyÂ
appreciated, as we believe that this sort() has some other purpose.
________________________________________________________________________________



For this mail we have listed only a limited number of doubts that we think are pressing. Based on further explanations that we might receive from you, we will send out another Âmail for the doubts that still linger.

Regards,
A-DRS.
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