Syncing Input And Output Block Sizes With Dd For Reliable Data Transfer

Mismatches in Block Size Lead to Data Corruption

When transferring data from one block device to another using the Linux dd utility, mismatches in the input and output block sizes can lead to corrupted copies and data loss. The core problem arises when the block size – the atomic unit of data transfer – differs between the input and output devices.

For example, if the input drive uses a 512 byte block size but the output drive uses a 4,096 byte block size, the mismatch introduces fragmentation and incorrect padding to blocks written to the output. This garbles the data, manifesting as errors like incorrect file sizes, unreadable files, or completely corrupt filesystem structures.

The onus lies on the user to explicitly match up the input and output block sizes used by dd. If left misconfigured, the flaws permeate all copies made with that dd command, risking backups and clones meant to serve as reliable data records.

Table of Contents

Explaining the Problem of Mismatched Block Sizes

To understand how input/output mismatches lead to unreliable transfers, consider what happens inside dd. The program sequentially reads a block of data from the input file or device, stores it in a buffer, then writes that buffer out to the output file or device before fetching the next.

If the output’s block size differs from the input’s, the write operation must realign these buffered blocks to fit the output’s blocks properly. This realignment pads out blocks with null bytes or fragments blocks across multiples chunks on the output.

For example, when writing a 512 byte block from input to a 4,096 byte output block, the extra 3,584 bytes come as null padding. Or if writing a 4,096 byte input block to a 512 byte output, the original block gets split and rewritten as 8 separate blocks.

This realignment arbitrarily restructures the data to fit incompatible blocks sizes without considering the underlying file format or structure. The resulting copies then reflect this broken alignment rather than the true input data, now rendered corrupted and unusable.

How dd Handles Block Size Mismatches

The dd utility features two key parameters that control the input and output block sizes used in copies – ibs and obs respectively. These define the atomic read and write sizes controlling dd’s core copy loop.

By default, dd picks a conservative 512 byte block size for both input and output. This attempts to maximize compatibility across devices using small blocks. However, it proves insufficient when I/O devices expect larger blocks, as on modern HDDs and SSDs where 4,096 byte or larger blocks now dominate.

If unmatched block sizes get configured via ibs and obs, dd transparently handles the realignment and padding required to fit these mismatched sizes. But the resultant corruption stems from this naive block remapping as it ignores higher level data structure and simply mismaps bytes between the unrelated input/output blocks.

Further, error checking code (like filesystem metadata) often assumes blocks represent atomic data units. The mismatch breaks that core assumption, confusing the error detection and triggering further system errors.

Synchronizing Block Size for Reliable Transfers

Ideally, the systems administrator first probes the expected, underlying block size used by both the input and output devices before invoking dd. Once known,supplying matching sizes to ibs and obs synchronize the block sizes to ensure reliable data transfers without costly realignment.

For example, reading from a modern hard drive expecting 4,096 byte block accesses, then writing to an SSD advertising native 4,096 byte erase blocks as well avoids any padding or fragmentation caused by a mismatch.

Setting the block counts via ibs and obs informs dd about the I/O devices’ atomic data size. Thus transfers avoid fracturing any structured data like database records or file formats into segments misaligned to the output device’s blocks. Matching them prevents broken buffer remapping that corrupts data copies.

Checking Current Block Sizes

The blockdev utility provides a convenient way to check reported block sizes for any attached Linux block device. For example, querying a HDD /dev/sda drive shows a 4,096 physical block size expectation:

# blockdev --getpbsz /dev/sda
4096

While the SSD drive mounted at /dev/sdb matches the same 4,096 byte block size:

  
# blockdev --getpbsz /dev/sdb
4096

With both drives verified as 4,096 bytes, configuring dd appropriately prevents mismatches going forward.

Setting Appropriate Block Sizes

Armed with the known block sizes above, we invoke dd with ibs and obs set to 4,096 bytes as well to maintain synchronization:

# dd if=/dev/sda ibs=4096 obs=4096 [..other flags]

This informs dd both drives use 4,096 byte atomic blocks for all read and write operations, avoiding misaligned buffers. Any file or structured data gets cleanly copied between the matched 4,096 byte blocks now.

Note if the devices featured different physical blocks sizes, say 4,096 input and 512 byte SSD output, the matching values get configured respectively instead.

Example Code for Setting Matching Block Sizes

Putting together discovery and matched dd invocation, a robust copy handles mismatched blocks with a short script:

#!/bin/bash

input_device="/dev/sda" 
output_device="/dev/sdb"

input_bs=$(blockdev --getpbsz ${input_device})
output_bs=$(blockdev --getpbsz ${output_device}) 

dd if=$input_device ibs=$input_bs obs=$output_bs conv=noerror,sync status=progress

This extracts each device’s published block size, saving as input_bs and output_bs variables. These then get supplied to dd’s ibs and obs parameters, synchronizing the copies to the drives’ expected atomic sizes.

Verifying Correct Block Size Configuration

With dd configured for matched blocks, the administrator should still double check for any truncation or padding induced by mismatches. Reviewing the output status after a dd run can verify synchronization.

No errors shown confirms dd did not need to fracture or realign any blocks during the copy. The elapsed timing also should match reasonable hardware transfer rates without slowdowns caused by padding huge blocks.

Likewise, confirm resulting file sizes match originals, and sample files exhibit correct headers and contents without truncation. Cryptographic checksums before and after aid this verification.

If mismatches still slipped through, iterative runs after tweaking block sizes helps converge on an optimal match of the underlying hardware. Sometimes device advertising lies about its actual block size, requiring testing different guesses until reliable copies emerge.

Avoiding Common Block Size Pitfalls

Administrators run into a few notable traps when struggling to match dd input and output blocks properly:

Trusting defaults blindly – The 512 byte default often undershoots modern drive buffers expecting much larger reads/writes. Verify via probing.
Misreading documentation – Some platforms like OSX report 512 bytes but use 4,096 byte internally. Test rather than assume sizes.
Bit vs Byte errors – Mixing up bits vs bytes leads to 8x mismatches. Double check base 2 vs base 10 bytes.
Ignoring alignment – Logical block addressing can still misalign physical blocks without compensation in partition tools.

Matching the sizes naively often requires tweaking to account for suboptimal defaults. Strange I/O stalls or out of space errors also hint at size mismatches falling outside expected drive behavior.

Summary: Matching Blocks Sizes for Peace of Mind

Reliable usage of Linux’s dd utility requires lockstep synchronization between the input and output block sizes. A mismatch induces data realignment that corrupts copies and risks data integrity.

Always check both device’s advertised block sizes, then feed appropriately matched values for dd’s ibs and obs parameters. This pairs up dd’s buffering to the underlying device expectations, ensuring clean data transfers that remain coherent and correct.

While matching input and output blocks takes a bit more legwork up front, doing so guarantees resilient dd usage without nasty surprises later down the road.