How does mirroring work between two non-empty disks?
I have a ReadyNAS unit in mirroring mode with two disks that were purchased new (empty): DISK0 and DISK1. I want to have an off-site protection in a bank safe: ideally I would like to purchase a 3rd disk (DISK2) and once per month replace DISK1 with DISK2. DISK1 will then go to the bank safe for a month.
A month later, I want to take DISK1 from the safe, take DISK2 from the ReadyNAS and insert DISK1 instead. Then, I take DISK2 to the safe for another month etc.
So I always have two disks that are mirrored, on is in the array permanently while the second is DISK1 for a month then DISK2 for another month then again DISK1 etc. So there is always a backup disk that is physically in the bank safe and never more than one month old.
1) When DISK1 is replaced in the array with DISK2, does the array mirror DISK0 to it? If yes, does it use any of the existing files on DISK2 and for example compares timestamps? Or does it just wipe out the disk and re-mirrors everything from DISK0?
2) If the array was destroyed would it be possible to rebuild the data from the disk that was in the bank safe at the time of array destruction? Meaning, if DISK0 disappears, can DISK2 be used with DISK3 (a new disk) in a new ReadyNAS, to rebuild all the data?
Re: How does mirroring work between two non-empty disks?
1) yes. it wipes whichever disk is inserted. 2) yes, potentially. but this is not a recommended way to back up, especially if you are pulling disks while the nas is operating.
doing as you suggest, in theory can work, however it places all disks in a greater wear and chance to crash during rebuilds.
to better back up, you should back up to external/usb drives, or do nas to nas back up.
Re: How does mirroring work between two non-empty disks?
Of course we don't know what your data consists of, but I agree with TeknoJnky that you should not adopt this as your backup strategy:
1. Your backup is difficult to read, other than in a readynas system. 2. The sata connectors are not built for repeated disconnection/ connection cycles, this may in theory shorten the life of the disks, as may the repeated volume rebuilds that are going to be required. 3. Your backup is potentially 1 month old when you need it - this may be ok for you of course. 4 Your backup is offsite, will you have an onsite backup for easier access?
If you can adopt the USB disk strategy suggested by TeknoJnky this would be better for an offsite backup, albeit requiring a 4th disk. You should consider if possible having an onsite backup.
Re: How does mirroring work between two non-empty disks?
Thank you for your answers, guys. My files are large digital photos (NEF and PSD, mainly). The offsite backup strategy is necessary to prevent loss of data in case of fire or burglary in the main office. It is unlikely but not impossible that all local (onsite) backups would then be lost so the offsite copy would be the only way to recover the data. The offsite backup would only grow 1 month old in case the disaster happens on the last day of the month; in average it would be just 1/2 month old so the loss would be fairly limited in my case.
I do have a local USB backup so I will just have to use this USB disk as the alternating off/onsite backup, based on your recommendations. The NAS will remain onsite only - and disks changed only to expand space.
Re: How does mirroring work between two non-empty disks?
Oops - I split off the Asustor posts (the digression seemed to be getting long), but then accidentally deleted the forked thread. My apologies. StephenB
Re: How does mirroring work between two non-empty disks?
StephenB wrote:
Oops - I split off the Asustor posts (the digression seemed to be getting long), but then accidentally deleted the forked thread. My apologies. StephenB
There are unlimited snapshots and backups 😉 Asustor discussion was definitely out of place here. The only relevant part was that commonly used Molex SATA and eSATA connectors are rated, depending on model, for max. 500-2500 cycles (and some SATA motherboard connectors are even rated for 50 cycles only!). This is very, very low, and indeed makes a solution requiring frequent disk removal highly questionable.
Re: How does mirroring work between two non-empty disks?
Yes. Though Asustor's MyArchive idea is using their regular drive trays - which like the ReadyNAS trays directly connect the internal disk's SATA connections to the backplane. They apparently are betting that the connectors will last longer than spec'd. I agree it is questionable, and (back on topic) don't think its a good approach. And of course in the case of RAID-1, the volume needs to be resynced every time a disk is inserted to make a new backup.
