> -----Original Message----- > From: Dan Williams [mailto:dan.j.williams@xxxxxxxxx] > Sent: Tuesday, October 6, 2015 11:32 AM > To: Song Liu <songliubraving@xxxxxx> > Cc: linux-raid <linux-raid@xxxxxxxxxxxxxxx>; Shaohua Li <shli@xxxxxx>; Neil > Brown <neilb@xxxxxxx>; Christoph Hellwig <hch@xxxxxxxxxxxxx> > Subject: Re: [PATCH 3/6] Enable create array with write journal (--write- > journal DEVICE). > > On Fri, Aug 28, 2015 at 4:27 PM, Song Liu <songliubraving@xxxxxx> wrote: > > Specify the write journal device with --write-journal DEVICE > > > > ./mdadm --create -f /dev/md0 --assume-clean -c 32 --raid-devices=4 > > --level=5 /dev/sd[c-f] --write-journal /dev/sdb1 > > mdadm: Defaulting to version 1.2 metadata > > mdadm: array /dev/md0 started. > > > > Only one journal device is allowed. If multiple --write-journal are > > given, mdadm will use the first and ignore others > > > > ./mdadm --create -f /dev/md0 --assume-clean -c 32 --raid-devices=4 > > --level=5 /dev/sd[c-f] --write-journal /dev/sdb1 --write-journal > > /dev/sdx > > mdadm: Please specify only one journal device for the array. > > mdadm: Ignoring --write-journal /dev/sdx... > > mdadm: Defaulting to version 1.2 metadata > > mdadm: array /dev/md0 started. > > Hmm, perhaps this should instead follow the same command line mechanics > as "--write-mostly" whereby all the devices listed after that flag take on that > role. Also, if we anticipate more roles I wonder if the option should be "-- > role=" that assigns the given role to the device(s) that follow. > > Thoughts? > > I'm looking at this from the perspective of reviving the SRT patches [1] where > a role for a read/write cache device is needed. Where a read/write-cache is > different than a journal. > > [1]: > https://urldefense.proofpoint.com/v1/url?u=https://lwn.net/Articles/59661 > 4/&k=ZVNjlDMF0FElm4dQtryO4A%3D%3D%0A&r=J3jGe56dIPfS5TN6DM82U > YbbeR1j2viaiSJI40tv6lE%3D%0A&m=dv5znFd%2BTZdHZaSKjzBVqJbdtNBALR0 > RgzhZsOmsWAU%3D%0A&s=80c812981ff98264cf1bb2b79032863d4e33ba100 > 093d41956eab3fbd33b5635 I think --write-mostly is used as _hint_ to RAID 1 that differentiates SSD from HDD. While --write-journal turns on "MD_FEATURE_JOURNAL" and specifies journal device. We should not need more than 1 journal device. In this sense, I think current --write-journal works well... Thanks, Song ��.n��������+%������w��{.n�����{����w��ܨ}���Ơz�j:+v�����w����ޙ��&�)ߡ�a����z�ޗ���ݢj��w�f
