Re: [PATCH v2] ext4: improve discard efficiency

[fengnan chang <fengnanchang@gmail.com>]

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On Mon, Jul 24, 2023 at 11:42 AM Guoqing Jiang <guoqing.jiang@linux.dev> wrote:
>
> Hi,
>
> On 7/19/23 17:36, Fengnan Chang wrote:
> > In commit a015434480dc("ext4: send parallel discards on commit
> > completions"), issue all discard commands in parallel make all
> > bios could merged into one request, so lowlevel drive can issue
> > multi segments in one time which is more efficiency, but commit
> > 55cdd0af2bc5 ("ext4: get discard out of jbd2 commit kthread contex")
> > seems broke this way, let's fix it.
> > In my test, the time of fstrim fs with multi big sparse file
> > reduce from 6.7s to 1.3s.
>
> I tried with a 20T sparse file with latest kernel (6.5-rc2+ commit
> f7e3a1baf).
>
> truncate -s 20T sparse1.img
> mkfs.ext4 sparse1.img
> mount -o discard sparse1.img /mnt/
> time fstrim /mnt
>
> 1. without the patch
>
> [root@localhost ~]# time fstrim /mnt
>
> real    0m13.496s
> user    0m0.002s
> sys     0m5.202s
>
> 2. with the patch
>
> [root@localhost ~]# time fstrim /mnt
>
> real    0m15.956s
> user    0m0.000s
> sys     0m7.251s
>
> The result is different from your side, could you share your test?

Here are my test steps:
1. create 10 normal files, each file size is 10G.
2. deallocate file：punch holes every 16k. The attached file includes step 1&2.
3. trim all fs.
So why does trim a new fs become slow? because with my patch,  in
ext4_try_to_trim_range
we need do alloc and free memory, this might cause 9us cost in
addition. So in current
version,  benefits can only be gained if there are multiple
discontinuous segments that
need to be trimmed in  ext4_try_to_trim_range.
This problem needs to be fixed, so I'll send another version.

Thanks.
Fengnan

>
> Thanks,
> Guoqing

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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <strings.h>
#include <sys/stat.h>
#include <linux/falloc.h>
#define _GNU_SOURCE 
#include <fcntl.h>
#include <sys/ioctl.h>
#include <linux/fs.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <time.h>

int deallocate_block_range(int fd, int start_block, int count)
{
    unsigned long start = start_block * 4096; // 以字节为单位描述块的范围
    unsigned long len = count * 4096; // 以字节为单位描述块的范围

    if (fallocate(fd, FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE,
                 start, len) == -1) {
        perror("fallocate");
        exit(EXIT_FAILURE);
    }
    return 0;
}

int create_file(char *file) {
    int fd = open(file, O_WRONLY | O_CREAT | O_TRUNC, 0666);
    // 打开文件，以写入方式打开，若不存在则创建，权限为666，文件长度为0
    if (fd == -1) {
        printf("Fail to create file!\n");
        return -1;
    }

    unsigned long file_s = 10*1024*1024*1024;
    unsigned long block_size= 1024*1024;
    unsigned long count = file_s / (64* 1024);
    char* block = calloc(1, block_size);  // 用'A'填充每个块
    memset(block, 0x3f, block_size);

    for (int i = 0; i < file_s/block_size; i++) {
        write(fd, block, block_size);  // 写入一个块
    }
    fsync(fd);  // 将缓存中的数据刷新到磁盘
    int off = 0;
    for (int i = 0; i < count; i++) {
	deallocate_block_range(fd, off, 8);
	off += 16;
    }
    close(fd);  // 关闭文件
    return 0;
}

int
main(int argc, char **argv)
{
	for(int i = 0; i < 10; i++) {
		char name[128];
		sprintf(name, "testfile_%d",i);
		create_file(name);
	}
	return 0;
}