sparse file performance (was Re: Is there a "make hole" (truncate in middle) syscall?)

Andy Isaacson
2003-12-05T21:00:24Z

On Thu, Dec 04, 2003 at 05:23:48PM -0600, Andy Isaacson wrote:
> On Thu, Dec 04, 2003 at 02:32:23PM -0600, Rob Landley wrote:
> > What are the downsides of holes? (How big do they have to be to
> > actually save space, is there a performance penalty to having a file
> > with 1000 4k holes in it, etc...)
>
> It's filesystem-dependent; some filesystems don't implement sparse
> files. The lower bound is one block; on extents-based filesystems like
> XFS it might be bigger. (If you've got 1GB of data, then a 1MB block of
> zeros, then another GB of data, you're probably better off allocating a
> single 2GB extent rather than two smaller extents with a hole.)
>
> There's no inherent downside to holey files; in fact they can be a
> straight-up performance win -- that's a block that doesn't need to be
> read from disk, just hand the user a COW pointer to your zero page. And
> if you're lucky and the preceding and following blocks are allocated
> adjacent on disk, you can do it all as a single streaming IO.

I got curious enough to run some tests, and was suprised at the results.
My machine (Athlon XP 2400+, 2030 MHz, 512 MB, KT400, 2.4.22) can read
out of buffer cache at 234 MB/s, and off of its IDE disk at 40 MB/s.
I'd assumed that read(2)ing a holey file would go faster than reading
out of buffer cache; in theory you could do it completely in L1 cache
(with a 4KB buffer, it's just a ton of syscalls, some page table
manipulation, and a bunch of memcpy() out of a single zero page). But
it turns out that reading a hole is *slower* than reading data from
buffer cache, just 195 MB/s.

200 MB file 234 MB/s (with warm caches)
1 GB file 40 MB/s (exceeds physical memory)
1 GB sparse file 195 MB/s

the 1GB sparse file was created with "dd if=file of=1gsparse bs=1M
count=1 seek=1023"; the filesystem is ext3.

Here's 'vmstat 5' while reading the 200MB file in a loop:

procs -----------memory---------- ---swap-- -----io---- --system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa

1 0 50968 4468 4872 410424 0 0 0 9 102 46 62 38 0 0
1 0 50968 4448 4892 410424 0 0 0 6 101 41 62 38 0 0
1 0 50968 4428 4912 410424 0 0 0 6 101 40 62 38 0 0
1 0 50968 4404 4936 410424 0 0 0 6 101 37 61 39 0 0
1 0 50968 4384 4956 410424 0 0 0 8 105 117 60 40 0 0
1 0 50968 4484 4984 410296 0 0 0 9 103 81 62 38 0 0

here's 'vmstat 5' while reading the 1GB sparse file in a loop:

1 0 55448 4460 2464 417320 0 0 217 6 144 3117 45 49 6 0
1 0 55448 4444 2480 417304 0 0 219 6 204 3237 50 44 6 0
1 0 55448 4444 2488 417288 0 0 218 9 181 3200 49 45 6 0
1 0 55460 4456 2468 417140 30 0 249 6 182 3193 46 48 6 0
1 0 55460 4396 2484 417300 0 2 220 12 140 3084 46 48 6 0
1 0 55460 4356 2464 417360 0 0 216 2 145 3101 47 48 6 0

The code is simply doing

while((n = read(fd, buf, sizeof(buf))) > 0) {
c += n;
for(i=0; i < n; i++) {
hist[buf[i]]++;
}
}

compiled with gcc 3.3.2 -O2.

Code appended.

-andy

#include <stdio.h>
#include <stdarg.h>
#include <stdlib.h>

#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <sys/time.h>
#include <fcntl.h>
#include <ctype.h>

static void
die(char *fmt, ...)
{
va_list a;
va_start(a, fmt);
vfprintf(stderr, fmt, a);
va_end(a);
exit(1);
}

double tod(void)
{
static struct timeval tv1;
struct timeval tv2;
double r;

if(tv1.tv_sec == 0) {
gettimeofday(&tv1, 0);
return 0;
}
gettimeofday(&tv2, 0);
r = (tv2.tv_sec - tv1.tv_sec) + (tv2.tv_usec - tv1.tv_usec) / 1e6;
memcpy(&tv1, &tv2, sizeof(tv1));
return r;
}

int main(int argc, char **argv)
{
char buf[4096];
int fd, i, n, m;
long long c = 0;
double t1, t2;
int hist[256] = { 0 };
unsigned char *p = buf;

if(argc != 2) die("usage: %s file\n", argv[0]);

if((fd = open(argv[1], O_RDONLY)) == -1)
die("%s: %s\n", argv[1], strerror(errno));

t1 = tod();
while((n = read(fd, buf, sizeof(buf))) > 0) {
c += n;
for(i=0; i < n; i++) {
hist[p[i]]++;
}
}
t2 = tod();
if(n == -1) die("read: %s\n", strerror(errno));

m = 0;
for(i=1; i<256; i++)
if(hist[i] > hist[m]) m = i;
printf("%lld characters read, mode at %d '%c' with %d\n",
c, m, isprint(m) ? m : '?', hist[m]);
printf("%f seconds, %f MB/sec\n", t2-t1, c / (t2-t1) / 1e6);
return 0;
}