qemu-kvmde 磁盘cache参数

qemu-kvmde 磁盘cache的参数有一下五种，在建立虚拟机的过程中 通过下面的函数设定int bdrv_parse_cache_mode(const char *mode, int *flags, bool *writethrough){    *flags &= ~BDRV_O_CACHE_MASK;    if (!strcmp(mode, "off") || !strcmp(mode, "none")) {        *writethrough = false;        *flags |= BDRV_O_NOCACHE;    } else if (!strcmp(mode, "directsync")) {        *writethrough = true;        *flags |= BDRV_O_NOCACHE;    } else if (!strcmp(mode, "writeback")) {        *writethrough = false;    } else if (!strcmp(mode, "unsafe")) {        *writethrough = false;        *flags |= BDRV_O_NO_FLUSH;    } else if (!strcmp(mode, "writethrough")) {        *writethrough = true;    } else {        return -1;    }    return 0;}这五种的详细解释如下：其中默认是writeback

https://www.suse.com/documentation/sles11/book_kvm/data/sect1_1_chapter_book_kvm.html