NDR / Every kind of NAS / NAS with SSD cache

Read-write cache · NVMe · Synology SSD cache · QNAP Qtier and cache

NAS with an SSD cache. The cache holds what the hard drives never received, and a dead cache takes it with it.

An SSD cache sits in front of a NAS's hard drives to make it feel faster. A read-only cache keeps copies of popular data and can be thrown away. A read-write cache accepts writes on the hard drives' behalf and flushes them later, and until it does, the newest data on the NAS lives only on the SSDs. When a read-write cache fails, the volume behind it either refuses to mount or mounts with holes where the last writes should have been, and the makers' dashboards say so in words that sound worse than the situation is. The cache SSDs are imaged with the set and their contents merged onto the reassembled volume. A set with its cache is £500 + VAT upwards after the free look, fixed in writing, 5–10 days at the bench.

Free first lookOne fixed figure in writingNo data, no bill on most jobsReturn postage paid

Rather talk it through? An engineer answers the bench line
0800 6890668

Power it down. Label each drive with its bay number. Do not rebuild, repair, initialise, reinstall or reset. A rebuild reads every sector of every surviving drive, and on a set with a second weak member it finishes what the first failure began. Nothing on the drives gets worse while the unit is off.

Cache symptoms, and what each means.

Not listed? Describe it on the form →

Cache SSD failed; volume will not mountDirty data in the cache. Send the SSDs with the drives.Do not delay

Dashboard offers to remove the cache and continueIt means continue without the last writes. Do not.Do not delay

One of a mirrored pair of cache SSDs is deadThe other holds the dirty data. Power down and send both.Do not delay

Recent files missing after a cache problemThe last writes were still in the cache.Do not delay

ZFS SLOG device failedRecent synchronous writes may be on it. Do not import without it.Do not delay

Packing it and posting it: power the unit down, write the bay number on each drive with a marker before it comes out, and send every drive from the set, including any SSD cache. Each drive travels in an anti-static bag inside its own padding, in a box with nothing able to move; the unit itself comes too if it holds an encryption key or a proprietary layout, and its power supply with it. Insure the parcel for what the files are worth rather than the price of the drives, and use a tracked service. The posting address is not printed anywhere on this site; it arrives by email in reply to the form, with a booking sheet to print; the sheet inside the parcel is what matches it to your enquiry when it is opened. Or hand the sealed parcel in at the nearest of ten drop-off points, your name on the outside and the sheet inside; say where you are on the form and it comes by email. We pay the postage home either way. The whole of it is written up on the guide to packing and posting.

How it is laid out, and what fails.

Read-only cacheCopies of data that also exists on the hard drives. Losing it loses speed, not files. It can be removed and the volume mounted without it.

Read-write cacheThe newest data lives on the SSDs until flushed. A failed read-write cache is a lost set of recent writes unless the SSDs are imaged and their dirty blocks merged onto the volume.

Mirrored cachesSynology and QNAP require a mirror for a read-write cache, which is why a single dead SSD is survivable and a dead pair is not. Both SSDs are imaged; the healthier one's dirty data is used.

Tiering and ZFS SLOGQNAP's Qtier moves whole blocks to the SSD tier rather than caching them, and a ZFS SLOG holds synchronous writes not yet on the pool. Both are part of the data and travel with the set.

What it says, and what it means.

Describe yours to us →

What you see	The usual reason for it	Where that leaves you
Volume cannot be mounted; cache failed	Dirty data in the cache	Cache SSDs imaged; blocks merged onto the volume
Remove cache to continue?	The dashboard offering to abandon the last writes	Say no; send the set with the cache
Recent files missing	Last writes were in the cache	Merged from the SSD images
Cache degraded; one SSD failed	Mirror lost a member	Power down; both SSDs sent
ZFS: pool cannot import, log device missing	SLOG gone	SLOG imaged and the pool imported with it

From the box arriving to your files going back.

Work we have closed →

Logged the day it lands, and the first look costs nothing Free

A case number goes on the parcel and a number on every drive the day it is opened, and an engineer settles what has actually happened before anything spins. The unit is examined on its own; each drive is assessed on our own equipment, never in a NAS that will try to rebuild. Back to you come two things together: a straight note of what is liftable and what is not, plus one figure, fixed and written down. Accept it, or decline and owe us nothing.

Nothing to pay for lookingA single figure, put in writingNo rebuilds, no repairs, no resets

The unit, and the drives, apart

The box and the disks are two different jobs. The unit is tested separately, because a dead power supply, a failed board or a bricked boot flash leaves the drives untouched more often than not. Each drive is then read on the bench: the ones that answer at full speed, and the weak or failed ones in clean air where they need head work. On units that hold an encryption key or a layout of their own, the unit is part of the data and is kept with it.

Unit and disks assessed separatelyFailing members to the clean bench

Every member imaged, once

The hard drives are imaged and the array assembled as for any set; the cache SSDs are imaged too, their dirty blocks identified from the cache's own metadata and written onto the virtual volume before the file system is mounted from it. Nothing is flushed on the originals.

Every drive, including the cacheWeak areas last

The set put back together, from the images

Order, chunk size, parity rotation and the reshape point are read from the drives' own metadata and the array is reassembled in software: mdadm and LVM under SHR, X-RAID and QTS pools; a ZFS pool imported read-only; Drobo's BeyondRAID zones reconstructed. The file system, Btrfs, ext4, XFS or ZFS, is repaired on the virtual volume, and encrypted volumes are unlocked there with the key you supply. Never on the originals.

Rebuilt in software, from imagesThe file system repaired on the virtual volume

You see the file list before you pay

What was recovered is listed for you first, and only then does a bill exist. Approve the list and it is invoiced; turn it down and it is not — and where nothing has come back, most jobs carry no charge at all. Recovered data travels home on fresh media bought in for your job, with the postage at our end. Your case is not closed until you have opened the files on a machine of your own.

No charge until you accept the figureFresh media, supplied with the job5–10 days at the bench

What arrives most often

Send the SSDs. A read-write cache is part of the data. A set that arrives without it mounts with holes.
Do not accept the dashboard's offer to remove the cache. On a read-write cache it means discarding the writes it still holds.
NVMe caches come out of their slots and travel in anti-static bags; the M.2 slot number matters as much as the bay number.
SSDs that failed are recovered as SSDs, which is a different bench and a different set of tools; the cache's controller is coaxed into giving up its blocks before they are merged.

One job, followed all the way through.

UK · NDR-2026-0544JOB LOGGED ✓

A Synology DS1621+ whose mirrored NVMe read-write cache lost both drives in a week

The volume refused to mount and the dashboard offered to remove the cache. The owner declined and powered down. The six hard drives and both NVMe SSDs were imaged; one SSD read cleanly, the other with its controller in diagnostic mode; the dirty blocks were merged onto the reassembled SHR volume and the Btrfs file system mounted from it. The week's work came back with everything before it.

100% of it recovered8 days here, and back by post

Illustrative example — replace with a genuine case

What helps, and what harms.

Do this much first

Power it down and leave the cache in place until the drives come out
Send the cache SSDs with the hard drives, labelled by slot
Say whether the cache was read-only or read-write
Send the unit; the cache configuration is on its board

What sets us back

Removing the cache to get the volume to mount
Flushing or resetting the cache
Initialising the cache SSDs
Mounting the volume elsewhere without the cache

Questions answered before you commit.

My NAS says the cache failed and offers to remove it. Should I?

If it was a read-only cache, you can. If it was a read-write cache, removing it means abandoning the writes it still holds. Power down and send the set with the SSDs.

Are recent files really on the SSD?

On a read-write cache, yes, until flushed. That is what a write cache is. They are merged back from the SSD images on the bench.

What if the SSD itself is dead?

It is recovered as an SSD, on the SSD bench, and its blocks merged onto the volume. The free look tells you what is there.

What does it cost?

The set with its cache is £500 + VAT upwards after the free look, fixed in writing.

How long does it take?

5–10 days at the bench.

Begin here if yours is doing the same thing.

Rack and SMB NAS →Four-bay RAID 5 and 6 →Synology →QNAP →

Nothing gets worse while it is powered down.

Looking at it is free. Back comes a list of what opened and what did not, together with a single price to finish, set down in writing while you are still free to say no. On most jobs an invoice only follows the data. Until that list reaches you, leave the unit off and the drives in their bays.

0800 6890668

Price my job Packing and posting

Every kind of NAS

Manufacturers

Whatever it is saying now

What it costs, and what follows