The short answer: I/O is the speed at which your account reads from and writes to the disk. When it reaches the plan’s limit there is no error: the account is throttled and the site answers slowly until the heavy job is over. It is nearly always a job, not visitors: a backup made inside the account, a large import, a compressed file being extracted, or a cache rewriting thousands of files. You find it by the time it happens, and you fix it by moving the job to another time or another place. What I/O is, next to CPU and memory, is in CPU, RAM and disk I/O in plain words; this article is the next step: bringing usage down and deciding whether it is time to upgrade.
Symptoms and causes
| Symptom |
Most likely cause |
| The site slows down every night, or every day, at the same time |
A backup plugin or a cron job compressing the whole site inside the account. |
| Slow while importing a database, products or a theme demo |
The import writes thousands of rows in a row, and through phpMyAdmin or a plugin everything goes through PHP. |
| Slow after uploading a large .zip and extracting it in File Manager |
Extracting writes every file in the archive, one by one. |
| Slow after clearing the cache or changing theme |
The cache rebuilding page by page, or a plugin preloading the whole site at once. |
| Slow after changing the thumbnail sizes |
Every image in the media library having its thumbnails regenerated. |
| Slow at no fixed time, and the account full of logs |
An error_log or debug.log that never stops growing: every error is a disk write. |
| High I/O at the busiest hours, with no job running |
Uncached pages reading many files on every visit, or bots crawling the site. Then I/O comes with CPU: CPU usage at your plan limit. |
Bringing I/O down, in order
| 1 |
Find the time: in cPanel, open Resource Usage, pick the period and look at the I/O graph (and the IOPS one, operations per second, if it is there). Note the hours when the line touches the limit and the faults. cPanel menu names change slightly with the version and the language: look for the meaning, not the exact word.
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| 2 |
Compare with scheduled jobs: in cPanel, Cron Jobs; in WordPress, the schedules of your backup, cache and security plugins. If the time matches, you have found it.
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| 3 |
Backups: our automatic daily backups already run every day. A plugin that zips the whole site inside the account repeats that work and takes up space too. If you want a copy of your own, send it off the server (off-site backups), make it less often, or back up only the database.
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| 4 |
Large imports: instead of phpMyAdmin or a plugin, import over SSH (importing a large database over SSH). On a shop, import products in smaller CSV files. Do it at your quietest hour.
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| 5 |
Big files: extract on your own computer and upload only what you need, or extract over SSH. Delete the .zip once you have checked everything works. Heavy downloads (videos, installers) are better kept outside the hosting.
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| 6 |
Cache and thumbnails: use a single page cache, limit preloading, and do not empty the whole cache without a reason. Regenerate thumbnails in batches, not the whole library at once.
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| 7 |
Logs: open the error_log, fix the error that keeps repeating, and only then delete the file. Turn WP_DEBUG_LOG off when you have finished debugging.
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| 8 |
Wait a few days of normal traffic and go back to Resource Usage. If the faults have stopped, the account is big enough; if they carry on with no job behind them, read the next table.
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Optimise or upgrade?
| If... |
The answer |
| The peak matches a backup, an import or an extraction |
Optimise. Moving the job in time or place fixes it; on a bigger plan the same job simply takes a little longer to hit the limit. |
| The cache or the logs write non-stop |
Optimise. It is waste, and it grows with the site. |
| I/O rises with visitors, with a well set-up cache and no bots |
Upgrade. It is real use, and bigger plans have more resources. |
| Several sites in the account and only one makes the peak |
Move that site to a plan of its own, or upgrade to a Business plan. |
| You need to process big files every day (video, huge exports) |
A VPS: disk and resources of your own, with no neighbours. |
Bigger plans
Each plan up has more I/O: from 10 MB/s on Pro to 50 MB/s on Business Ultra, and more IOPS, the operations per second, from 1,024 to 8,192 (table below). Your account’s real limit is the line on the Resource Usage graph. You upgrade in My MozOut, under Services, on the service, at Upgrade/Downgrade; you pay only the difference until the renewal, and the site, the e-mail and the files stay where they are: how to upgrade your plan. For heavy jobs every day, a VPS gives you resources you share with nobody.
| Plan |
Disk I/O |
IOPS |
| Pro |
10 MB/s |
1,024 |
| Plus |
15 MB/s |
2,048 |
| Ultra |
20 MB/s |
3,072 |
| Business Pro |
30 MB/s |
4,096 |
| Business Plus |
40 MB/s |
6,144 |
| Business Ultra |
50 MB/s |
8,192 |
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Do not stop an import halfway to take the load off the site: the database can be left with half the data. Let it finish, or restore a backup before running it again. And before deleting old backups or big files, download them to your computer (making a full cPanel backup).
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Got a usage warning about I/O? Look at the time first: a peak every night at the same hour is nearly always a scheduled job, and it is fixed without changing plan. What each warning says is in usage warnings at 75%, 85% and 95%.
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Cannot find what makes I/O spike? Send us a screenshot of Resource Usage and the time of the peak, through a support ticket in My MozOut.
Open a support ticket
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