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Subjectinotify limits - thousands (tens of thousands?) of watches
Hello everyone,

First, apols for using up bandwidth, but I honestly found no other place
where I can ask about this (and get meaningful reply).

I'm not a kernel programmer, but I want to develop a program that would
watch modifications in *all* user directories on a busy server using
inotify.

This is for high-availability purposes - events would be collected and
once every several minutes changed dirs would be rsync'ed to failover
server or smth like that would be done.

As inotify watches particular directory and not its subdirs, I would
have to watch all directories.

This means I would have to create thousands or even tens of thousands of
inotify watches.

So my questions are:

1. is it safe? that is, will it not lock the kernel up, or cause
excessive memory consumption?

2. is it economic in terms of CPU time and RAM? I have no idea how to
even measure such a thing happening in the kernel..




Here's first test take on inotify watch (runs on Debian):

######################### cut ################################
#include "stdio.h"
#include "stdlib.h"
#include "errno.h"
#include <linux/types.h>
#include <linux/inotify.h>
#include <sys/syscall.h>

#define EVENT_SIZE ( sizeof (struct inotify_event) )
#define BUF_LEN ( 1024 * ( EVENT_SIZE + 16 ) )


static inline int inotify_init (void)
{
return syscall (__NR_inotify_init);
}
static inline int inotify_add_watch (int fd, const char *name, __u32 mask)
{
return syscall (__NR_inotify_add_watch, fd, name, mask);
}
static inline int inotify_rm_watch (int fd, __u32 wd)
{
return syscall (__NR_inotify_rm_watch, fd, wd);
}

int main( int argc, char **argv )
{
int length, i = 0;
int fd;
int wd;
char buffer[BUF_LEN];
fd = inotify_init();

if ( fd < 0 ) {
perror( "inotify_init" );
}
wd = inotify_add_watch( fd, argv[1], IN_ALL_EVENTS );
length = read( fd, buffer, BUF_LEN );

if ( length < 0 ) {
perror( "read" );
}
while ( i < length ) {
struct inotify_event *event = ( struct inotify_event * ) &buffer[ i ];
if ( event->len ) {
if ( event->mask & IN_CREATE ) {
if ( event->mask & IN_ISDIR ) {
printf( "The directory %s was created.\n", event->name );
}
else {
printf( "The file %s was created.\n", event->name );
}
}
else if ( event->mask & IN_DELETE ) {
if ( event->mask & IN_ISDIR ) {
printf( "The directory %s was deleted.\n", event->name );
}
else {
printf( "The file %s was deleted.\n", event->name );
}
}
else if ( event->mask & IN_MODIFY ) {
if ( event->mask & IN_ISDIR ) {
printf( "The directory %s was modified.\n", event->name );
}
else {
printf( "The file %s was modified.\n", event->name );
}
}
else if ( event->mask & IN_ALL_EVENTS ) {
printf("Some other event on %s, mask %x.\n", event->name,
event->mask);
}
}
i += EVENT_SIZE + event->len;
}
( void ) inotify_rm_watch( fd, wd );
( void ) close( fd );

exit( 0 );
}
######################### cut ################################
Regards,
mk


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